Is it illegal to post a software without permission? I am making a software which would do gridcomputing (i.e combine many computers processes to speed up a Function/App/Computer) I would post it online like a virus and spread to computers. This software will not take any of data. Delete it. Use it. It will only use a little bit of internet and 10MB of RAM.
Unauthorized use of a computer is illegal in most of the United States, and in many circumstances it is a federal crime. Here's a round-up of applicable laws from the National Conference of State Legislatures.
This would be unauthorized access to a computer. The offence is found in Section 1 of the Computer Misuse Act 1990. Note that this is a criminal offence, approaching the police about it might be the best way of handling the situation (if you want to go that route, and also, I'm not a lawyer).
If you don't want someone copying your stuff you don't need a licence at all - that is the default way copyright works. Simply put a copyright notice in the deployment and any splash screen or about screen.
From the perspective of US law: Scenario 1: Removing the watermark is perfectly legal, obviously. You own the copyright. Scenario 2: Abandonware is not really a thing in the US when it comes to copyright (and the Wikipedia article you linked says this.) Even if the company went out of business, the copyright almost certainly went to somebody - the former owners, the creditors, etc. As far as the Internet Archive goes, they are taking advantage of an exemption in the DMCA's anti-circumvention provisions, along with the copyright exemption found in 17 USC § 108. But both of these exemptions require you to be a "library or archive" to take advantage of them, and you are not a library or archive, so you cannot take advantage of either. As a practical matter, if the software is really abandoned it's unlikely that anyone will actually come after you for infringement, but that's not quite the same as it being legal - you could be on the hook for up to $150,000 in statutory damages per work infringed, if the owner does sue you. You could attempt to claim fair use, of course. If the image is abandoned you might get a favorable result on factor 4 of the fair use test (the effect on the market for the original) since there's no market if nobody can find the owner. But it would depend on the other factors; that alone wouldn't get you fair use. Scenario 3: I'm not sure where you get the idea that downloading a copyrighted image to your desktop is somehow protected. It isn't automatically legal to save a copy of something just because it's on the Internet. And the existence of a watermark may indicate that someone wanted to prevent the file from being copied. The website containing the image will likely have terms of use, which may indicate whether or not you're allowed to make a copy. For example, Stack Exchange's TOS says in part: Subscriber may download or copy the Content, and other items displayed on the Network for download, for personal use, provided that Subscriber maintains all copyright and other notices contained in such Content.
If it was illegal to make the entire copy, it is illegal to copy half the file. See also, e.g., Basic Books v. Kinko's Graphics Corp., 758 F. Supp. 1522 (S.D.N.Y. 1991) In response to the question of whether moving it to someone else's servers makes a difference: it is the act of copying, not the possession of the copy, that is the violation of copyright law. As for the copy a service assists someone to make, contributory liability would be the issue. You don't need to be the person making the copies to be contributing to the infringement--material contribution to the copying process and knowledge of infringement run a serious risk of creating contributory liability. If seriously considering such a service as a business model someone would need to consult a copyright expert with technical knowledge or pay counsel with technical knowledge for a good bit of time to do some research. There are major civil and criminal consequences if the industry or the government decides to go after a service doing this, so legal expenses would be a significant and important cost of doing business that would significantly increase barriers to entry. In addition, the service would likely be in violation of its agreement with storage vendors, who would want to minimize their legal exposure and might well discontinue the service when they learned what it was doing.
Generally, such sanctions prevent certain sorts of transactions in goods and services with nationals or entities of the nation under sanction. The exact list of transactions prohibited or restricted varies. If Open source software were being provided as a service, so that the recipient paid directly for a license, or for customization or configuration work, or for some sort of consulting or assistance, such transactions could be banned or restricted by a sanctions regime, but might not be. (If the sanctions included that particular class of transactions.) However, if it is merely a matter of an open source product being published, for anyone to download, install, and use, I don't see how that would be barred or restricted by any sanctions of the sort recently in use.
Use of SE is subject to the terms and conditions specified here. Section 3 says "You agree that all Subscriber Content that You contribute to the Network is perpetually and irrevocably licensed to Stack Exchange under the Creative Commons Attribution Share Alike license." That means that your contributions can be used forever. That means the stuff is there, end of story. Section 11 on Termination says "Stack Exchange may also terminate, block, or suspend any and all Services and access to the Network immediately, without prior notice or liability, in its sole discretion, for any reason or no reason at all, including but not limited to any Subscriber breaches of any of the terms or conditions of this Agreement". That means you can be banned. So, yes, what they did is legal.
It's a contract violation if you're under the EULA. It may be a contract of adhesion, but such "clickwrap" contracts been found to be acceptable and enforceable in software EULAs out of necessity. However, there may be some limits. If you're not under the EULA, as you argue, then you lack a license to use the software at all and it's an outright copyright violation and/or a theft or misappropriation of the software. Whether or not you can be sued depends in part on what you do with it — if you don't release the material or otherwise cause damages then there's not much to sue for... Added for clarification: to answer the framing question, supposing neither contract or copyright applied, one could be sued in tort or in equity (i.e. for unjust enrichment).