“Never underestimate a person willing to die for a cause.”

Unknown

There are a number of variations of this to suit a variety of contexts/situations:

“Never underestimate your enemy but always make him underestimate you.”

“Don’t underestimate me. I know more than I say, think more than I speak, and notice more than you realise.”

“Let people underestimate you. That way they will never know for sure what you’re capable of.”

“There is no greater danger than underestimating you opponent.”

“I never over/underestimate my opponent, but I never over/underestimate my talents.”