Well, I don't think I can help you not feel SAD. I always feel sad when I slaughter an animal. When I shoot a deer I usually blubber like a baby for a few minutes. I still feel a bit sad about the cornish that we processed a few days ago. I liked them, they were sweet, simple birds. I also felt sad, but less so, about the nasty guinea cocks, which were perhaps more like your roosters. It's okay to feel a little sad. That means that the gravity of taking a life is still with you, as it should be.
I do not mourn them the way I mourn the loss of a beloved pet or animal working partner, though. (I am mourning the death of my first SAR dog still, five years on.)
But guilt is a different emotion. Guilt is what we feel when we believe we've done something wrong.
To feel less guilty, contemplate the life of your mean rooster vs. the life of every industrially-raised chicken you've ever bought wrapped in plastic. (Warning: this could end up making you feel guilty about buying and eating commercial chicken, so is not without its perils.)
Assure yourself that your roosters have had a good life, and that you care for them so much that you will personally ensure that their deaths are as distress-free as possible. Then make it so -- review all the good advice here on humane slaughter, and mentally rehearse the way you are going to do it. If you make a mistake, fix it quickly and move on. Understand that it still beats being trucked to a processing plant after seven weeks of a miserable existence.
Give thanks to the birds, to their spirits, and to whatever deities you credit with their presence on the planet. Approach the slaughter with a sense of grave mindfulness. If prayer is part of your belief system, now is the time. Dress them carefully. I find that it helps that not one bit of the bird goes to waste here. The dogs eat the heads, feet, necks, lungs and giblets. The innards go into the maggot-bucket for the benefit of the other birds. The blood (put some water into the catch-bucket so it stays liquid) goes onto the nitrogen-greedy vegetable crops. The feathers go into the compost. Everything is reclaimed. I can mollify myself with the assurance that we respected the gift of the bird as thoroughly as possible.
Here's a thoughtful blog post from a writer who was faced with a tough call on a pet rooster:
http://www.dolittler.com/2009/05/23/The-REAL-“omnivore’s-dilemma”-notes-on-my-first-slaughter.html