What she's thinking:
"Humans are overall stronger than monsters."
"Even human kids can hurt and kill monsters."

What she forgot:
"I'm a boss monster."
"I shoot fireballs."
"I'm the fucking strong" (Her strength is more or less on Undyne's level, so be grateful she simply didn't do a 100-yard toss with you whenever you climbed down the stairs)

Seriously, she simply knocks Asgore aside with him at full power (with his titanic stats, a mere -4 for being caught flat-footed would mean nothing).

That, or she got caught up in the "prove yourself" moment, only to have a sudden realisation that she just killed a confused young child who just wants to go home, and only just met her less than a day (in-universe) ago.