So less regulation allows banks to use their clients money more aggressively. Then why would banks need to borrow so much money? And if its returned overnight,

So less regulation allows banks to use their clients money more aggressively. Then why would banks need to borrow so much money? And if its returned overnight, then why is FED keep pumping more and more? Can something bad for market come out of it? Like one of big bangs bankrupting or something?