Well run through the problem line by line:
tskill hl2 - Does this end the app you wanted it to?
tskill GameOverlayUI - Does this end what you want it to?
I see no reason why they shouldn't, so I'll presume they are working correctly.
The next thing you want to do, is run your game.
'START' is the correct command to run here, but Instead of just running the .exe for the game, you're trying to process the launch through steams own application launcher?
I don't have steam myself, but I was under the impression that this was unnecesary. Back when I had CS installed, I could just
Code:
win + R > cs > enter
(due to the environment variables I had set up.)
I know you need to login to Steam, but if Steam is already running (you haven't tskill'ed it anyway), then why don't you just START the .exe itself?
If you are logged out of steam, then just add another line onto the BAT that logs you back in.