I remember when i used to play cait.. idk how i played her enough to get 100k mastery, but eventually i got allies like you who would rage about traps. Idk why i just never got that used to using her traps. Sometimes I would put one under a rooted target but it would be slightly off and not snare em lol. Sometimes I would be out of position and run up to snare but it was too late and I would get raged at. You would probably think i was doing it intentionally apparently. Ease up man. Hindsight is 20/20. In the heat of the moment, not everyone executes. Did this happen in silver or lower? Because if youre in silver, thats the kind of inconsistent play you should expect. Anything lower and you should expect even less. Even gold isnt that good. If youre plat or higher then yeah, thats unacceptable at that ELO.
So, why should he be banned? Was he verbally abusive towards you? If so, then yeah that will count against him, but you know you dont just get banned straight away from one game of toxicity with no ZT phrases. Did he grief you? Ehhh not landing traps isnt objectively griefing, maybe seeing the game footage would show more, but obviously cant just take your word for it. Intentional feed? Id have to see the game but 12 deaths is not indicative of inting. From your description he was not griefing, especially considering he even gave a reason that indicates he would have trapped them, but was not in range.