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(I'm not sure whether your comment was trying to soften the Russian invasion of Ukraine or not, but any attempted justification is no justification at all).
Same way the US used Iraqi mobile labs to justify its invasion and the seeking of regime change in Iraq, with arguably more deadly to civilian tactics like "Shock and Awe".
Moreover, if biological weapon will target Russians only (by asking them a question in the Russian language, I suppose) then it's completely legal for Ukraine to use such biological weapon to defend itself against Russian aggression. Madness is an important part of MAD.