Overkill was a pretty bad lightgun game, peopel might've been satisfied aesthetically with the whole obvious fake exploitation cinema parody direction (not a fan personally... original HOTD stuff was every bit of Italian Zombie exploitation as Overkill was without a fake look to it). But oh boy what a mess what a mess. The yneed to play the original HOTD games a bit more to see how you do your enemy placement, not a fixed camera (yeah move around your view, worthless) horrible and that unsignificant scoring system, numbers for the sake of numbers blah.
Rather see HOTD4 port, while departing from the usual HOTD gameplay, it is actually pretty good, not virtua cop 3 good, but good enough. Larger Zombie quantity and control your uzi with bursts so that you could headshot, reload is also slightly trickier, harder to count the bullets you've shot. Panic bombs are nice addition, it already kinda feels like a shmup now that zombies come in overwhelming forces.