It honestly depends, I feel that verbal communication should be an option, even if it causes more trouble than not. The thing is, if you are in a team with hot heads, and the enemy team has hot heads, chances are they are going to attack each other and everyone else, it's in their nature. League is as much a psychological game as it is a literal one, from the moment you are selecting your roles and champions to the last twenty seconds of the game, its as mental as it is physical. Your heavy focusing of a fed target is a completely legitimate strategy. Some might argue that is as far as you should go, and as long as you don't degrade them harshly i.e. mom jokes, discussing their personal life or history - or at least what you think - details, their life choices, I consider it valid.
If League didn't want anyone cross-chatting, there would be no /all function in the in-game chat. Riot has restrictions on what you can say because your desire to win should not cross the threshold beyond human decency, but trash-talking is a legitimate strategy, meant to evoke rage and frustration in order to expose weaknesses and bait opponents into favorable situations. If your opponent is volatile enough to be baited into diving you under your turret and then dying after trash-talking, you've won that bout of the psychological war aspect of League.
People associate all the trash-talking with the worst of what League has to offer, but that isn't the case because there is a difference between someone who talks smack, and someone who is toxic. A smack-talker wants to win, and he knows he can run his mouth and piss you off, and make you play without a calm and cool head, and he can make up for a lack of skill on his part by making your own suffer by your own emotions. This is legitimate play, and if they can intelligently do this, then I give them a kudos for being ruthless in how they achieve victory.
Someone who is toxic often doesn't see it this way, he is just venting his frustrations and just wants everyone else to be mentally and emotionally hurt because they have been, or because their ego has been damaged and they can't handle their lack of success. A toxic individual throws insults around, often to both their allies as well as the enemy team, because they know that words can hurt, and they care at that point to do nothing but hurt you as much as they can.
