The US was sending old equipment. To replace it they were and spending money on R&D and manufacturing of current and next-gen equipment for the US military, not Ukraine. The US got to observe and study the use of it's older equipment in an actual land war against a geopolitical adversary. The war has demonstrated US equipment superiority over Russian equipment.
The US did not send any troops, only old equipment and some money.
Regarding "hopes to end the killing". The killing can end today if Putin withdraws. He is the aggressor who attacked without any justification or provocation. If somebody invades your neighbour's apartment and starts looting and hurting them, do you just say "give up, it's easier"?
Some might but those are not people we respect.