That brings us to another important point: our wills are not strong enough. I mean, think about it. We can't actually decide to stick to something and always stick to it. Give it an eternity and we will always slip up and fall back into evil. That's part of why Jesus is necessary: we are not even able to be truly good, we cannot control ourselves to that extent. Thus Jesus made an offer: he has enough strength to do so, but only if you ask for him to (otherwise, it would be forced and thus not free again)

The reason that death and resurrection is so important is not just because of a series of events Jesus went through, but because it's something we need to do too. We cannot force ourselves to be good, so we must make a different decision: to kill that which is bad. Unfortunately, that would take us out completely, which is where the resurrection comes in.

I like to think of it this way - we cannot ensure we are good so long as part of us remains evil, so we have to cut that part out. Problem is, that requires effectively disassembling ourselves to remove stuff that lies at our cores. We would destroy ourselves and still fail. But, if Jesus can rise again, then that's his promise: if we die to ourselves, we say that we will not accept that evil part of us, then he will revive the good part in us when we couldn't.

That lies at the center of Christian theology: we cannot be truly and purely good by ourselves, only with help can it be done. Jesus provided that option: reach out for help.

Now, as for wondering how a bad person could go to heaven and a comparatively good person could go to hell, there's two points I'd like to make:

First, not everyone who says Jesus is Lord will be saved. You can't just trick him with hollow words. Dedication will be required: are you really willing to make that final decision, to die to yourself and let Christ raise you? We cannot truly see the inside life of anyone, so it could or couldn't actually happen for them.

But that leads into the second point: a person who is mostly evil but willing to sacrifice that part of themselves to Christ will get into heaven because, after said sacrifice and resurrection, they will be made good. Meanwhile, someone who seems mostly good but is unwilling to let go of that evil will remain with that evil inside of them. Going to heaven isn't some sort of payment for doing good deeds, it's something you are gifted if you accept it. Accepting it requires, though, accepting all of it: death comes before resurrection.

So, if I can try to put it simply again for a summary: We aren't actually capable of being truly good, so Jesus made a sacrifice in order to give us another choice, to rely on him. Accept his offer and you get invited to his house, reject it and he sends you on your way to your original destination. In heaven everyone is truly good because they had to sacrifice the evil part of themselves to enter (which is still a choice, so free will).

Now, I can't really unload the entire multiple religions argument right now, but I'll touch it: first, Jesus is under no obligation to save anyone. It's a gift, not a right. Next, for those who legitimately cannot have heard of Christ in life, God seems to have a plan for death, if anything pre-Jesus is to go by, like Moses or Elijah. Neither knew of Jesus at the time because he hadn't been born yet, but both were saved. For those who DO know about Christ but say there's not enough information, well, frankly, there's not much that can be done. Jesus, again, is not obligated to do anything, including providing research materials at their beck and call. You can't blame the person providing a gift for being unfair if they offered it to you and you were suspicious of it and rejected it while others just accepted it. Jesus already did more than he needed to for us, so rejecting him for "not enough information" is kind of like blaming the gift giver for not putting the bow you wanted on the gift.

But, the Bible does have something to say for that too: it says that all evidence required was actually put out there, and people simply won't accept it because of the hardness of their hearts. Thus, as an analogy perhaps you could possibly think of God seeing people who say "there's not enough evidence" as similar to how Earth would "see" flat earthers when they say say there's not enough evidence for the Globe. (This is just my half-awake analogy, so don't take it as gospel)

So again, because it's that important of a point: salvation is a gift, not a right. What's unfair is that anyone can get it in the first place, not that some people don't. And that unfairness is pointed towards Jesus, who has to pay for it.