Good game, but with flaws
The mixture of game styles works well and I really like the storytelling. It's dark and engaging. Despite the shotcomings it was fun to play.
The artwork is terrible, but you acknowledged that. So I guess that will improve.
The game overall is too easy. The Ryan levels especially are just mashing the attack button, there needs to be some skill involved. Different kind of weapons (upgrades) for each three would be nice. And of course different enemies (stronger zombie types) could make it more intesting as well.
In the first and second Thomas level I got a "failed" screen twice, even though I was at full health. After clicking retry my gun wouldn't fire so I had to refresh the game.
Scrolling in the Ryan levels was bad, you shouldn´t have to walk against the edge of the screen to scroll. In the Thomas levels the scroll box was too small as well. Either keep the screen centered on the character or put the scroll box at around 33%.
In some levels I missed some dialogue because I was clicking the mouse to shoot, and the last zombie died while I was still clicking, which made me skip the dialogue. Maybe putting a small timer after the last enemy would fix that.
Overall, a good first effort!