It is. You kill people, both in Serial Killer and all those ther games I mention. In fact, I am wrong. You kill more people in those games than you do in this game.
It's not the killing. Killing stuff is just fine, it's how you approach it. Shooting things is what every FPS is all about. Killing hundreds of uniformed soldiers is just fine. Uniformed soldiers aren't real people, it's okay to shoot them. And it's okay to nuke England or blow up a star system in a 4X game, because you never see the millions or billions of people who die. They're statistics, not real people. And while virtually every roguelike features mass murder as a method of keeping score and character advancement, this one here goes out of its way to personify the designated victims.
So if you like mass murder, why you're bothering with this, when you have a wide variety of different games giving you the same exact same thrill (murder)?
You'd likely kill less people in this, but those people are portrayed in a drastically different light than other games, which is why this thread became so long and caught the interest of so many. In GTA, half the people you kill are enemies that you're fighting against for your own interests, not for simply killing them. When you run over people, you do it for the fun of seeing them fly into the air, ragdoll-style. In Assassin's Creed, killing civilians damaged you, and killing enemies was alright, even if they had families, because you had to save the world. To repeat what someone else already said, the fun/thrill of this game isn't going to stem from the killing itself. It comes from the way the game treats it.
There's a wide variety, but this one is forging on new ground. Hence the interest, and why people are 'bothering' with it. Soadreqm explained it well.