Ugh. Jesus bloody Christ. That was as corny as a... um... as a very corny thing.
"I am a returning soldier. I have lost everything. I have nothing to lose. I have nothing to live for... except revenge."
In the name of decent writing, give me a break.
Then again, this is Prototype after all. Expecting some degree of cleverness or subtlety is probably an exercise in futility.
...
Oh well, at least the people behind this game seem to have recognised that Mercer is a unrelatable dick and have relegated him to the role of villain. That's a step in the right direction... right? Although, as the OP suggests, no doubt it will turn out that he's just misunderstood and then Mercer and the new kid team up against a greater evil etc etc.
Lastly, storyline wise, I suspect the reason Mercer would inject other people with his virus would be to spread the truth of the events of the first game by lending out his memories and those of all the people he ate.