Downing a civilian aircraft with a SAM battery, or MANPAD, near an active conflict, is galaxies apart from planting explosives on civilian airliners.
And I don't mean legally speaking, although it is, I mean they aren't even in the same universe when talking about blowback, politics, military responses, threat management, PR, escalation ladders, etc.
As risky and escalatory as it is, I can at least understand using freight airplanes to deliver incendiary packages to shipping warehouses.
I'm not saying I think it's good, but I can at least piece together the rationale for such actions from Russia.
The same cannot be said for blowing up civilian airliners.
Just from a realpolitik perspective, domestic support for military aid to Ukraine is broadly down across the voting populace in most, if not all, of Ukraine's biggest ($$$$) partners. Eventually that will likely result in the election of candidates who reflect that view.
Want to know the fastest way to not just immediately reverse that, but have 75%+ of the voting populace support radically escalating Western involvement? Blow up one of their civilian airliners.
Shit, blow up a French airliner and I'd say it would be coin flip whether they deploy active duty military ready for combat operations, in theatre, within a month.