These are very true, and yes we shouldn't be mean to friends.
But sometimes I do say mean things.
And when I say mean things, I always feel that I've already been holding back my full strength (i.e. if I didn't hold myself back, I can kill you with my words).
I've received advice from mother, colleagues, friends, and even one-night-stand pal (yes, I did extend my anger to this far), that I should try to hold back my tongue and try to calm myself before I vent out my anger, or else I'll do harm that cannot be reversed, try not to let my emotion control me, etc.
I agree, but what I didn't quite dare to admit, is that actually, I'm fully aware of what I'm doing, which means, I'm actually saying those things on purpose with an aim to hurt! When I feel that I'm on fire, I'm been enduring the other person for quite a while, I don't like them and I've been holding back the angry for quite a while already, or even worse I look down on them for long. I just try to not to let that out due to courtesy. When I have been enduring that anger or that energy for long, people will feel it even I try to hide it. This is pushing friends away but sometimes I don't really care that they go. I feel safe and peaceful when I don't have to maintain the shallow politeness with them. There is an old Chinese saying that 'The interaction between gentlemen is as bland as water' (very rough translation). This is true - when you keep contact with someone too closely and too frequently, the weaknesses of each other are nowhere to hide, thus arguments will appear.
It took me a really long time but I'm still struggling with this problem. Or is it really a problem?