I'll admit that anyone who beats up a baby puppy is sick, but you shouldn't go to prison for 1 1/2 years for being sick. People have gotten less for worse things.
Using your logic I should be okay next time the little 6 year old brat down the street starts throwing rocks at my house. After telling him over 25 times to not throw rocks I will just snap and beat him up to stop it. I must be sick to think like that and in your book I don't deserve jail time either. I'm sure his parents will accept my apology and move along without any concern. /sarcasm
"Sick" implies it's not his fault.
He didn't get 18 months for being sick. He got 18 months for committing a crime that is 1)heinous in itself and 2)strongly indicative of sociopathy (i.e. the only thing that will keep him from doing the same to the first human who annoys him is fear of punishment).