It would only correlate with mental instability if I was unable to recognize the disproportion, I can see it's disproportionate, cruel, and wrong. I'd do it anyway and wouldn't feel the least bit bad about it after. But I'm in control of that - it's not mental instability, no red haze, just conscious decision.
I'm a no half measures kind of guy, I'm completely unafraid to cause a great deal of pain to those who have wronged me - maybe you don't have the stones or you've never been so connected to your fellow humans eh?
There's no risk of imprisonment because there's no proof of the crime, who is going to believe an educated man with a good job and no priors waterboarded and beat a some scumbag burglar off the estate with a phone book for a couple of hours before phoning the police? No one.