Do BB care if you renege a non IB internship to become an SA at their bank?

They won’t care.

Won't care - e.g., JPM doesn't care

Will it look bad if I renege on an offer?

According to the Ivy Research Council, 47% of students who had exploding offers had fewer than two weeks to accept the offer. While companies should expect you to make a timely decision, we think that is far more reasonable during the traditional fall recruiting season when you can consider all your options. If an employer is enforcing a deadline that makes you uncomfortable, consider whether that company's culture is truly a fit for you."

https://careers.jpmorgan.com/us/en/how-we-hire/faqs

Scroll to the bottom - look at the last 2 questions 

Would love to hear what others think about this