The thing is your punishment is all on you. You are making it seem like one has to be greater than the other when both offenders can be in the wrong. In this case i would say both people are in the wrong. People shouldnt be trolling, but also the people affected by the troller should know how to act civil about it. We cant do much with these people aside from reporting them. Call them names, wishing death/cancer on them or a family member does nothing to help the matter, so why even bother?
They only show your chat because that is all on you. You chose to say/type those things and you are responsible for your actions just as the people who are trolling your game is accountable for their actions. Then we have the fact that one of these offenses are easier/faster to detect and in turn punish than the other. With toxic behavior in chat all you need is the chat from a couple of games in order see how they are in game.
With "trolling" its much more difficult. Are they actually feeding, and if they are is it intentional? some people have such a vague word with that definition that such small things like giving up First Blood should mean the person is feeding, or trolling. It could be a bad match up, or just a bad play. They thought they could go all in, but ended up dying. Maybe they are on tilt, or just arent in their A-game. Hell the person could be lagging like hell and we all know Riot's servers arent the greatest. People cannot play perfectly 24/7, people are going to make mistakes, and sadly people do abuse the report for these little things, and we all know people love to exaggerate on things with this game.
All im trying to say is that in most cases both parties can be to blame. It doesnt have to be one or the other. The person shouldnt be trolling in the first place, but that is what happens when you have a popular game like this. People thrive off of making people lose, or knowing they are going to say something in chat. They know how the system works and will use it to their advantage. And sadly people will fall for it. This is why we have the term: Don't feed the Troll. They want that from you and will report you because they know if anything bad happens to them, you will more than likely get punished first because its easier to detect.
But that goes the same way with people who are toxic in chat. I assume most people should be old/intelligent enough to know why these peoples are doing what they are doing. The internet isnt anything new and things like this have been going on long before league was probably even thought up. But people dont deserve to be talked like that even if they deserve it. You should be the smarter than that and just ignore them. You dont give them the attention they want they will either leave defeated, or go on full blast which just makes your report even more valid. You must also do your part in actually reporting people. Most people when dealing with these people dont even report them because they just jump right into their next game, or just leaving quickly. Also i assume writing in the description helps the people looking over the reports to see exactly why you reported them.