Ics does it on its own... go into running apps by going into manage apps, running, you'll see that your device keeps most apps killed already.
I'm going to suggest that you don't use an app killer.. it can cause issues.
The reason is that it could indiscriminately kill an app that's needed at the time, and cause force closes and other issues... excessive battery usage by having to kill and restart apps over and over. Over all device instability could result in extreme cases.
But its your choice