Well, when you click Fiora's rapier he says "I settle my scores" so it makes you think he killed her. Then again there's the fact Fiora in one of her emotes she draws teemo and calls it a pretty picture so that could draw an assumption she doesn't hate Teemo.
When you click Mundo's syringe he says "Size is a liability" so it could mean either his or Mundo's size screwed one of them over or maybe Mundo died and Teemo didn't because he was too small to be hit by whatever killed Mundo?
"I've done things im not proud of" is also something he says. So it's possible he killed them all but didn't want to. "Turns out I have a proficiency at killing" could've just went on a rampage killing things without being able to stop himself.
Tons of things you could say there. Then there's the trist deal. Trist and Teemo are supposedly best friends and he said "We aint all makin it out of here alive" maybe he killed her or he escaped something and she didn't? Maybe he had to kill her when they were on some duty together and he didn't want to but needed to.
I'm hurting my brain let me stop