It's a shame, this game would have been amazing, but it has so many glitches that its just frustrating to play. You get thrown out of the map very often, and because the game cant render too many enemies and corpses at the same time, the game lags and its pretty much impossible to aim and even shoot in these situations. I know these problems wont be fixed because this game is over a decade old, and its really a shame, since its otherwise the best game in the series and I wouldve loved to finish it if the game was in a playable state.
Edit: I tried the game out on crazymonkey and found that it played much better there. After beating it, I've got to say that its one of the best flash games I've played. The presentation is amazing, the gameplay is fast paced and intense, the soundtrack is good and enhances the action, and the game is overall a massive improvement from its predecessors. The only complaints I have are with some of the levels in the lab. The zombie level takes way too long to clear, and bashing zombies with a pipe for 10 minutes straight just gets boring eventually. There's also a glitch in the final boss that makes it impossible for enemies to spawn, forcing you to melee it into submission. Lastly, the ending is a little unsatisfying considering all the effort it took to get there. Despite this, I really enjoyed this, and I cant wait to start playing the other games