But he told The Seattle Times that instinct took over when a thin man in a beanie cap, dark clothing and sunglasses pushed a black backpack across the bank counter on Tuesday and demanded money.
Nicholson threw the bag to the floor, lunged toward the man and demanded to see a weapon. The man bolted for the door with Nicholson in pursuit.
He chased him several blocks before knocking him to the ground with the help of a passer-by. Nicholson then held the man until police arrived.
On Thursday, Nicholson was fired. Key Bank spokeswoman Anne Foster declined to comment on Nicholson and his actions.
Police and the FBI discourage such heroics. Bank tellers are trained to get robbers out the door quickly and are advised against possibly escalating a situation over money that's federally insured.
Nicholson said he understands why he was fired.
"They tell us that we're just supposed to comply, but my instincts kicked in and I did what's best to stop the guy," he said. "I thought if I let him go he would rob more banks and cause more problems."
Seattle police Sgt. Sean Whitcomb said the best course for citizens is to be good witnesses to crimes.
"When confronted by a violent criminal, it is best to comply unless they feel their personal safety is in jeopardy. It is possible that taking action and confronting the criminal may lead to the injury of the victim or other bystanders."
"You want tellers to be proactive, but you want them to do it safely," said FBI Special Agent Fred Gutt.
The would-be robber, a 29-year-old transient, has a lengthy criminal history, including convictions for theft and robbery, according to court records. Charges in the attempted robbery were not immediately filed.
Nicholson said he has run after shoplifters while working at other retail jobs.
"It's something I almost look forward to. It's a thrill and I'm an adrenaline-junkie person. It's the pursuit," he said.