Wow. That's bold.

"See, your honor, by stabbing the victim 30 times in 45 seconds, I was trying to ensure that his death was as painless as possible. I should get some credit for that, right?"

Well, that would be grounds in some states to counter the prosecution's argument that a convict tortured a victim, so yes.

Is concealing or destroying evidence of a crime an aggravating factor under the guidelines?

It always depends on state law, but as a general matter, no. Destruction of evidence is not by itself an aggravating factor under any death penalty statute I am aware of. That doesn't mean, however, that conduct which includes the destruction of evidence will not be admissible at sentencing as part of a broader aggravating factor argument.

"I wasn't trying to destroy evidence, your honor, I was trying to limit the emotional shock when the victim's family found him."