To have never killed a friend who was a bit too inebriated. I'm guessing he probably waited until his pal sobered up a bit.
“Thus Beowulf bore himself with valor;
he was formidable in battle yet behaved with honor
and took no advantage; never cut down
a comrade who was drunk, kept his temper
and, warrior that he was, watched and controlled
his God-sent strength and his outstanding
Beowulf (as translated by Seamus Heaney) lines 2177-2183 || Italics mine