Awesome game you have here! Fantastic idea to have different difficulties. A simple idea people often neglect to add in. The controls are fairly smooth in regards to movement and fighting.
There's lots I love about it, but a few things I think need to be addressed. There are far too many melee weapons. You should really only have a few. Especially for how useless they become in the later levels (at least the very weak ones). The reload time seems a little long as well, but that would be a bit more balanced if you could swap your weapons faster.
The onslaught is way too much since it takes forever to reload and you can't outrun them. I had some of the better weapons and still didn't stand a chance. Like some others have mentioned, the behemoth is severely underpowered. Also, not sure if intended, but I found I could melee zombies through walls before they even entered the house.
Overall, I really enjoy this game and I think you're on to something really great! Keep up the great work!
Edit: I have replayed this a few times over now and I have yet to get past the onslaught. Even with 3 good weapons, it's just too much.