This is just my personal opinion, so please by all means down-vote me if it makes you feel better.
Assuming you were playing a normals match, I myself wouldn't mind getting advice, but the thing is... it's a video game. Unless the person asked for it, they probably don't want it. They're just trying to have fun. Not to mention, I'm sure that person knows what they are doing wrong already, so reminding them of that will more than likely frustrate them when they're already trying their best.
Now, assuming you were playing ranked, then these players should have already experienced massive amounts of toxicity just from playing ranked games before, and thus should be able to handle something phrased so nicely in comparison to other rude shit I've seen said in ranked matches before.
In this specific situation, what set the Malphite off probably wasn't just your comment, but rather the supports follow up, in combination with the frustration of having to fight against Lulu as a melee champion. He was more than likely already trying his very best to fight against her, and was already so frustrated that your guys' comments set him off. Did he over-react? I believe so, since he should already have prior experience with this type of negativity. However, understand all of his circumstances in that match and it will make more sense that he did what he did. Amumu is pretty much the same issue here; He already knew he made a mistake, was already frustrated, and his own team's comments set him off. Yes, these are overreactions, especially if these games were ranked matches, but understand that both of them were probably already frustrated and it simply set them off.
What I would suggest in the future if you really want to criticize someone for what they are doing (which I am not saying is bad whatsoever, but rather it's not the best thing to do in these sorts of situations), then try to give them advice in a fashion that's more positive. Say something like "Hey Malphite, I know you're probably very frustrated with your lane right now, but may I suggest that you build x-item against her?" Or to the Amumu: "Hey Amumu, thanks for trying to engage, but is it possible that next time you could maybe wait a little longer until we're all confident we can follow-up?" This will not always work; these people might very well _still _ AFK due to their lack of control over their own emotions. However, you will at least be able to safely say that you did everything you could to try to help out your team.
