"I should also note there is a sniper of full ammo on the roof. I presented my suspicion of him, and he replied with "Nah brah", and then I found out he had a sniper for his primary. So already I'm about 80% sure it's him, but I leave anyways knowing I don't have enough evidence. I return about 5 seconds later and I found a sniper in a different spot on the roof, and the one full of ammo missing. I pick up the sniper and find 5 shots missing. Right now it's pretty obvious that it's him, but I still can't kill him because I haven't caught him red handed."
-He could of started shooting random objects (like I do) when bored. Swapping weapons is not a sign either, some people realise that they are at a disadvantage etc regardless of team.
"I recall playing on a dm_island and I found a body in the sewer, spattered with blood all along the walls, and unidentified. His last words was "I'm with X". I instantly said "Kos X", and the admin addressed me in that little text that pops up on screen saying "You can't use last words that would be RDM", to which I responded with "It's obviously him, look at how closed in this area is and stuff" to which the admin replied "He could have suicided, or been randomly calling out to joke around." It was pretty damn obvious that he didn't suicide considering the last weapon, and there was the fact that the person never ID'd the body. However this still wasn't enough for the all might rules."
-Clearly you should choose your servers better.
"Also a lot of servers like to give T's extra weapons (ak47, silence tmp, etc) that are either useless, make it easier for T's to go on a rampage, or are just plain overpowered as fuck (rocket launchers, etc). The later just make it easier for T's to win, but completely lack that whole "Someone is coming to kill you" idea. I mean I recall there being times when the traitor would literally call himself out, and go on a rampage with said overpowered weapons and typically getting a very large amount of kills. However, the rampage issue is a problem that can be created by the players themselves."
-See above post
There is really a lot of TL'DR in your post.
Games are full of dumb people. If you play on a server long enough you will begin to understand which servers have better players etc. Best stick to those ones.