Every time I try to launch dark messiah, it starts the usual "preparing to launch Dark Messiah" popup, then switches to "Validating...", goes to 100%, then stops. Every time. I have never been able to play it.
The demo launched fine (out of steam) and so does the multiplayer, but not the single player.
This is not a minor problem, and if it's not fixed, gypped me out of $50.

Try contacting steam support they should be able to help you with this issue:
http://forums.ubi.com/eve/forums/a/tpc/f/8821027804/m/2361074195

I had this problem with the single player demo version. If you manually validate it before launching it, it seems to solve the problem.
Right click on the entry in steam > properties > Local Files tab > verify integrity of game cache.

It'll start the validation process (though it took a while to validate for me with the demo). For me it actually updated something in the files, but was launchable after.