If you really want to do a multiplayer mod, go ahead and do it. Just like unknownsoldier said, if you feel bad about getting the idea from deadpool then just give him a little shout-out in the release topic and/or readme.txt that the idea for the mod was based off of his. And besides it's not like multiplayer is that creative of an idea to steal... if it's an idea anyone could of easily came up with on their own (like multiplayer) then I don't think it should be considered stealing to try and make your own. If it's a really creative and unique idea, maybe get permission from the original author first.
Like everyone else said, don't let AfterLife get to you. He's trying to hurt you and you let him win by giving into him. AfterLife is banned for good from here so you don't have to worry about him here, and just ignore him on other places, simple as that. WE all support you (just look at this huge topic of support), so for us, say "fuck AfterLife!" and keep pushing through.
About my tutorial/mods being open-source... I make them open-source for a reason. I want everyone to be free to learn from them, modify them, or do whatever to them freely as they choose. I will never be upset if someone takes a mod of mine and re-releases it even with slight changes, in fact I'll be honored and it'll make me happy you even wanted to do it. I don't care if you give me credit or not, since honestly in most cases people will probably know it was mine originally anyways, and even if they don't all that matters to me is that the mod I put out there is making people happy since that's the only reason I release mods anyways. One of the most rewarding parts of modding for me is seeing everyone enjoy the mods, or watching people play them, and so forth. So go ahead and take my mod, make it better, and re-release it. Won't hurt me in the slightest and it'll just give everyone else another fun mod to enjoy.