Question
if I didn't use enought water, will weed--be-gon kill my grass?

Answer
I have no idea how you used that product. If it is a crabgrass killer, it is probably going to kill the grass anyway.
I don't use weed killers, and I have a weed free lawn;
I am on a completely organic program.
If there was a recommended amount of water to use, and you didn't, it will probably damage or kill your grass.
Hope this helps you some
Charlotte