DS has just turned 5yo and has recently started saying "I hate you" lots. He uses it basically when he has been slighted or upset in some way - however small.
So for example, I'll tell him not to snatch a toy from his younger brother (2.10yo) but to share/play nicely/be kind, and he'll react by shouting "I hate you DS2".
Or he'll get cross at me for saying he can't do something, or if I've raised my voice slightly after asking him 4 billion times to wash his hands, and he'll shout/cry "I hate you! I loved you before but now I hate you". The other day at bedtime I was sitting on his bed and he said "I hate DS2". DS2 shares a room and was in the bed next to him.
I feel this is within the realms of normal, and I of course know that he doesn't really hate me/DS but obviously in that moment he feels he does.
However, there are times when it does upset me, as it sounds so horrible coming from the lips of a 5yo. And he seems to use it so casually and without much provocation. It doesn't help that DS2 has started to copy him and they now get into "I hate you", "no I hate you" exchanges which are horrible 
Depending on the situation I'll respond along the lines of "well, I still love you, and you still have to wash your hands" or "you might be cross/frustrated/annoyed at DS2 but I don't think you hate him/can you think of a different way to tell me what you mean as hate isn't a nice word to use/can be hurtful" etc. But sometimes I end up getting a bit cross with him and will say something like "that's a horrible thing to say/stop using that word/you're hurting my/DS2's feelings"
I guess my question is what's the best way to deal with this phase? (I hope it's a bit of a phase...). Do I ignore the more 'casual' uses of it as I suspect he may be testing/pushing boundaries, and just deal with the episodes when he's clearly upset/angry?
Help!