Is it normal that i try to justify people's behaviours?

Absolutely. Working out why people have acted how they have acted is not only the fair thing to do, it'll help you be more successful and well-liked in life for all kinds of reasons. People who are frustrated or unpopular or feel like society has rejected them often find that they can't, won't or just don't care enough to know how other people think.

However, like you, when I think I've intuited why someone acted like they did, I've usually had to make all kinds of assumptions, generalisations, and deductions that may not be relevant for the individual. I think I'm right more often than I'm wrong but I think I'm wrong to a degree that would be unacceptable if I was being judged on it (I think I'm wrong something like 30% of the time and partially wrong something like 80% of the time).

"You never really understand a person until you consider things from his point of view... until you climb into his skin and walk around in it." - Harper Lee

It make you a better and smarter person to consider all potential perspectives. The trick is not to assume you know, but to keep an open mind. A lot of the time you don't even need to know if you're right or wrong, you don't always need to know someone's motives. Mostly you just need to decide how you feel and what you need to do to make yourself happy/satisfied in the situation.

same here, i always like to try and see the good in people, and that there's always a good explanation. my ex gf hated this about me, didn't like that i stuck up for people that she would rather i not.

no i don't. but i hate people who blame everything on others i just can't stand those types of people.

I do that too. It makes it really difficult to get angry at anyone.

It is nice trying to find a reason behind everyone's actions. However, if you try to analize everyone you will get crazy. Sometimes the justifications are too deep to be understood. Other times, people are just cruel... Don't think much about it...

I usually do that as well.

Just be careful people have a way of taking advantage of your good nature.

Depends on their attitude.

If they aren't a huge asshole then I will try to give them the benefit of the doubt and think "well, this person was under a lot of stress" or even try to justify it with something I have done myself "I'm no better, I have made the same mistake".

But if they're a huge asshole I don't care. Being an asshole to me will always change my mind away from your favor. Unless you're a funny asshole. That's different.

I still do this regarding a friend who stole a large sum of money from me.