I hope that every reader of these columns had the same reaction when McComb suggested traveling back to 1994 to kill Officer Walker before he became Agent Walker. This is a serious anomaly akin to killing Adolph Hitler: if you succeed, you will never have any reason to make the trip, so barring some application of the common understanding of Niven's Law, it leads inexorably to an infinity loop.
We can be grateful that he failed.
Note--and this is important--that although Agent Walker came to that time and place to prevent changes to the past, and did reverse numerous changes, he is not responsible for having saved his own life. We saw the version of history in which McComb's thugs shot him twice in the chest and left him for dead on the lawn, but his body armor protected him and he survived.
At this point, too, we commend Timecop for recognizing an aspect of replacement theory often overlooked: the stacking of variant histories. The history we are observing is not the original history in which the Walker family lives in the big old house outside town, nor the final history in which once again they live in that same house. We are seeing an intermediate altered history--a version of events created by the trip McComb makes to the past, and then undone by the trip Walker makes.
Having said that, we must mention that there is then a very common problem, one that we first noted in connection with Back to the Future: the Agent Walker that exists knows nothing of the history through which he actually lived, remembering the history that now never happened. In undoing the history McComb created, he should undo the version of himself who lived through that. He will not be surprised when the car delivers him to the house in which he has always lived, and his wife who never died comes to greet him along with the nearly ten-year-old son they have been raising for most of the last decade (whose name he knows, along with the boy's favorite ice cream, favorite toys, books, and television shows, and all the other things you come to know about your children).
Further, there are still at least two more rather significant problems in the film, still to be addressed.