Just to add a few points to this debate, hopefully in a balanced way:
While Gunn's case is admittedly rather extreme and gratuitous, it's worth noting that almost everyone has some aspect of their past that they'd like to forget. To draw on an even bigger and more integral example within the MCU, would it be fair for Marvel to not have hired Robert Downey Jr. because of his somewhat checkered history? This was actually a debate that arose within the studio at the time Iron Man was being developed, but thanks to Jon Favreau fighting for his choice, the MCU hired their arguably most important and popular actor. Downey's issues were no more family-friendly than Gunn's, but it would hardly have been fair to pass him over for something that happened in the past, which he made successful efforts to correct and atone for. I think we should give Gunn the same benefit of the doubt here.
However, the above being said:
I do see your point here; depending on Gunn's social circle at the time, it might be plausible to say he was influenced by certain behaviors and attitudes that don't reflect his true self, but such a large amount of comments of that nature is rather distasteful, and I can sympathize with the fact that it was too much for some people. Personally, I was able to see the context behind some of the jokes, although I'd, hopefully, never even think to make comments like that in the same scenarios. I do think that these tweets deserve scrutiny, but it seems that they have been given a lot of thought over the years, and that Gunn has learned from his actions, which is the best that can be hoped for.
I also don't know if I can entirely buy this. As a society we have been dealing with the impact of words a lot lately, and the question of when words cross a line does need to be asked. Can we say that threats of violence that are not enacted are "just words?" I'm certainly not one to be offended by every little thing someone says, and I actually find that our culture is much too sensitive in certain cases, but there does need to be a line somewhere, and even if Gunn didn't cross it, he came pretty darn close.
I don't think this assumption applies to Hollywood only. Everyone is capable of lying, but just because they are, it doesn't mean that we should take everything someone says as a deception. James Gunn issued an apology for his past behavior many times, and since his first apology it seems that he has mostly avoided the actions that he was guilty of. That is as clear an indication as we have that he was telling the truth when he said that he was remorseful, and that the things he said do not reflect his character. Patterns can indicate character, but a broken pattern is just as telling.
That's about the best way I could put it. I don't think people should be forced into feeling like they're walking on eggshells when they speak, because that flies directly in the face of the ideals we've created as a free society. People should be able to say what they want, no matter what, so long as it does not threaten violence or degrade other people. There is a massive problem brewing online where people of certain opinions--both popular and unpopular--are being censored or excoriated for some things that they say, which are in many cases certainly less offensive than the tweets being discussed, and that should stop. However, it is always prudent that we think about what we're going to say when we're online, as we have the wonderful benefit of being able to edit our words before we post them, and we should use it whenever we can.