I'll be honest, I just don't see the big deal.
When the Teen Titans came back from Infinite Crisis after One Year Later, one of the coolest ideas I thought Geoff Johns had was to introduce Wendy and Marvin to the team as super geniuses who help the team with their technology. I thought that was a clever way to work in two of the supporting characters from the 1970s' Superfriends cartoon series.
So yeah, if you asked me, "Hey, Brian, should we kill off Marvin and Wendy?" I would say, "No, I like them."
That said, I don't see how killing them off is much of a problem, especially in the fairly clever way that Sean McKeever did, by having a stray dog (who is named Wonderdog, wink wink nudge nudge) turn out to be something along the lines of Cerberus, therefore introducing to the team the son of Ares, who is going to be a major villain for the Titans.
When I get irked about death in comics, it usually involves one of the following:
1. Sexism - Not an issue here
2. Any form of minority misuse - Not an issue here
3. Killing off important characters just for "punch" (a la Karen Page and Mysterio biting the bullet to add some drama to Kevin Smith's run) - Not an issue here
4. Killing off good characters that other writers wanted to use (a la Sue Dibney, Max Lord, Ted Kord, etc.) - Not an issue here, as they weren't even being used in Teen Titans much, which is the only title they appear in!
So that really leaves the whole "you're killing off characters from the SuperFriends, which is a kid's show! That is wrong!" And I don't buy that because, first off, the SuperFriends was a cartoon show from over thirty years ago. Secondly, Wendy and Marvin have not been around SINCE then. These characters are not important characters. Kids MIGHT recognize them from re-runs of the SuperFriends, but for the most part, they're recognizable just to older comic book fans.
They're Jean DeWolff, essentially.
Now, you can always say, "I like them! I don't want them to be killed!" which was my reaction to Jean DeWolff dying - I liked her as a peripheral background character. That's fine - you don't have to agree with a character death (*cough*Nomad*cough*) to allow that the writer has to have some leeway with who he/she kills off.
You have to allow writers to kill off peripheral background characters, especially when their deaths are part of a long-term storyline (as was Jean DeWolff's death - and yes, even Nomad - and is clearly the case with McKeever here).
So don't worry, Sean McKeever, I don't think you're evil!