Difference between role-play reasons and made up reasons

[Stealther]

I was in a server and I was thinking that you know you need a RP reason to lets say kill someone right? The reason has to be that they actually did something to you like destroyed your house etc, it can't just be like they did nothing to you but you kill and say your reason was "He killed my family" even though he didn't, right? I was just checking so that I can play the game knowing all the rules and that I won't do something stupid.