Can I stop a run that is already going?
Yes. Any live run can be cancelled by its owner, from the run screen. A cancel is terminal and it counts as nothing: no failure review, no demotion, no mark against the workflow. A person pulling the brake says nothing about whether the process works.
Usually the problem is not one bad run. You spot a hardcoded address that is wrong or a filter that matches everything, and cancelling runs one at a time is a losing race against the trigger. So the confirm dialog offers to stop every live run of that workflow and pause the workflow itself, ticked by default. One action, because it is one decision.
Cancelling bounds what a run can still record, not what it has already done. A request that has left for someone else's API has left. Everything the run tries to write after the cancel is discarded, so nothing further is recorded, but the brake cannot reach into a call that is already in flight.
You can say why afterwards. The dialog asks what went wrong once the run has already stopped, because stopping should never wait on you typing.