What do you do when you feel like killing someone, anyone, everyone?!

I'm having the afternoon-evening from hell and I'm getting angrier and angrier to the point of feeling like that or something. My parents are out and my grandmother is being a complete burden - behaving like a little child wanting small things all the time the minute I'm back to doing something and then shouting when I take more than one minute to go to her. Then I got an important phonecall and voice mail telling me to call back but that person is unreachable and I'm here going mad and as if that wasn't enough I'm either coming down with a cold or have already started my hay fever phase (don't know which is worse) and I'm feeling like crawling under the sheets and forget about it all. Only thing is, I can't

Sorry, needed to vent, but seriously, what do you do to get rid of this nagging feeling?!