Nik, so sorry to hear you've had a bad day...I'm sure you're quite heartbroken over the dogs that were euthanized, and also over having your cats at the shelter right now. Sounds like you're stressed out, and Colin isn't doing much for you in the sensitivity department at the moment.
I know you're upset, and I also know you love Colin a lot...I don't think I could tell you to leave him, or that I think the situation will only get worse, and all of the other stuff. I'm just someone reaching out to you from the web, and only YOU are living the day-by-day stuff with Colin...you are the only one who can follow your own heart about that kind of thing.
Anyway, couples fight, people we love say things they don't mean (and we do this too!), and sometimes aren't quite there for us in the way we would like them to be...no relationship is perfect, and sometimes it takes some soul-searching and a bit of forgiveness to make things doable. Compromise exists in every relationship, and the need for communication. I'm just going to leave it at that.
And, with all of that being said, I think you should wait for the air to clear a little, give each other some space today(long enough for you both to handle this kindly, and without flared tempers), take the time to do something for YOU (like a nice hot bath, a nap, curl up with a blanket in front of the tube, a glass of wine or cup of coffee, paint those nails, whatever works for you), and then, calmly tell him that as much as you'd like to go with him, you sense that he's stressed about something, and that you've indeed had a stressful day, and that you've decided that most reasonable solution would be for you to stay home with the dogs, while he goes to Alabama. This way, there won't be any cost due to a critter sitter, and you would feel better knowing that they're safe and secure inside. If for some reason, this sets him off in some way, and he behaves like a horse's patoot about it, you can simply say, "Well, you've just made my decision even easier." AND STAY HOME WITH THOSE DOGS.
Think of it this way...he'll be gone, you can have some you-time, and it may be a nice little break from each other. Every couple needs a little "single" time now and then, even if it's for two hours, or 3 days. There's nothing wrong with that, and it can be a healthy thing for both of you!
Chin up, Nik! Don't let anyone treat you like a doormat. The minute you begin sensing that you're being stepped on, Colin will need to be informed. Hold your head up high, and approach everything calmly, and with dignity, when you're able.
Plus, you'll have a great time hanging out with us while he's gone.