Well...I've read this some time before in one of the websites and ever since...I can't help remembering it whenever I lose my temper and end up being nasty and feel bitter myself. It always, always helps to be nice to people, however annoyed you are with them. Patience is a virtue with bound-less rewards, even if you don't see them right now. Just overcome whatever that feeling is you've towards that other person - jealousy, hatred, anger whatever. Remember - you are being the bigger person if you can maintain your calm Umm..we all know that itchy feeling after a fight I guess - even if we've won technically, even if we were right - just because you were nasty "during" the fight, it somehow doesn't feel like you've won. It just feels like you've lost something.
Most importantly, never be bad to someone who is approaching you for help. Just understand that you can make their life atleast a tad bit better if you can overlook your bitterness. Even if they don't say it, you know they're going to be grateful. And who knows - Life is strange. You never know when you'll need that person's help. And you'll just feel elevated knowing that you've helped someone inspite of your negativity towards them. Imagine - otherwise you're just going to end up being frustrated and bitter, unknowingly trying to justify to yourself why you are sad.
And believe me - It has never helped me till now whenever I've lost it and started being nasty or cold or unhelpful. Not when I had to fire my cook or when I yelled at my dad. It's always good to break the ice. If you feel that you're justified, just express it calmly and don't be nasty. I know it's hard and may be not always possible - But why wait to learn things the hard way? It's good to keep remembering it whenever you want to be angry or nasty for the heck of it. And don't show your own irritation on others. Just be nice and helpful - always.