In the experiment, a group of students played James Bond 007: Nightfire
Well that explains the players' reactions then. They were relieved when they died because it meant they got to stop playing that awful game for at least a moment. And, when they killed an enemy they felt anger or anxiety because they had to keep playing it.
One thing which is very misleading about this study is that it didn't test players reactions versus playing against real people as opposed to the computer-controlled enemies. If they had, they would have found the players' reactions as a whole to be the opposite of their findings.
Another misleading thing about this study is that this game was played on a Gamecube which is hardly the typical platform for FPS games.