I see where you're coming from, but I think this meets the threshold of 'pre-planned'. Proud Boys were apparently openly bragging about smuggling guns into DC ahead of the protest. Their leader was banned from DC two days ago amid (ultimately realised) fears violence was planned. Hell, IEDs were found in the Capitol. You don't just happen to have an IED on you - you make that ahead of time. There is a pattern.Ttg17. Agreed.
However without splitting hairs the Westminster bridge incident someone planned and executed a pre planned plot to murder people.
In this case a spontaneous set of incidents carried out by a collective set of idiots seemed to escalate and mob mentality inspired people to do things perhaps they wouldn’t normally do. I know nothing of that woman so this is guesswork and I accept I could be wrong, but I’ve a hunch she didn’t wake up yesterday morning planning to murder someone and did as a “martyr”.
The individual woman who was killed maybe didn't pre-orchestrate every action she took that day. But she was part of a pattern of pre-meditated violence.