Even before the serum, Blonsky carried himself as a guy who enjoyed a good fight and was willing to inject weird stuff into his body to get stronger. The serum obviously amplified those feelings and he went off the deep end, but he was already a bit of a sociopath before then.
That said, General Ross was to blame for introducing a super powered Blonsky to the world. It's amazing he didn't lose his job or even go to prison. Instead, he ended up promoted years down the line.