I'm so sorry you lost your precious Brendan. I think you know that so many of us understand, not only the loss of a beloved pet, but the feelings of guilt, regret, or second guessing. I'm sure your emotions are raw now, and in your pain, you are putting blame on yourself. Please try not to do this. Easier said than done, of course, but know that in time you find comfort in the memories of truly happy and safe and loved he really was. He would want you to celebrate that love.
And please come and talk whenever you need to. It really does help.