In terms of deaths, it'd be Seung Hui Cho, but Adam only injured 2 employees by shooting them, the other 26 were slaughtered very quickly. I'd personally say Adam is the deadliest. I'm sure he would've killed far more if his gun didn't jam.
“This is it. This is where it all ends. End of the road. What a life it was. Some life." - Seung Hui Cho