what can i do to feel better?

Do 你 have a fence in your backyard 或者 anything? If not, 你 should really get it. If 你 do, then that's trespassing, and he isn't allowed to just take a family member of yours (I count pets as family members, for those of 你 confused) and bring him/her to a pound. That's just cruel. Also, if 你 can bring some proof to someone about what 你 think/believe/know he's doing, then the Humane Society and/or ASPCA should step in. That's animal cruelty and is frowned upon; I'm sure it won't be let go just because they'll see it as his "first offense" 或者 anything. Even without anything, just warn them of his actions, so that they can monitor him 或者 any 动物 that are nearby in that area.

I know that you're angry with him, but calm yourself down and think about other things. Don't fear over what might not even happen to your new dog; stress isn't good for the body 或者 mind. Take a few breaths whenever 你 get mad. If that doesn't work for you, try screaming in a 枕头 或者 something of the sort. Overused (and somewhat ridiculous), I know, but it works. Talk to your parents about it, too; communication is a very good thing. Maybe they can suggest something to help 你 as well.

I know what it's like to lose a pet; I'm very sorry for your loss. I may have not had a dog, but I've had two 猫 and a fair handful (if even that much) of goldfish. ...And a frog... >_> It sucks, I know, but they're in a happier place. I'm sure that they would have wanted 你 to live out life like you've been doing, and not have to worry about them. You've watched over them, so now they can watch over you.

I hope that 你 get better.