Ok first of all she literally ate a moon with Billiam Milliam and his minions on it in the finale so yes she has killed people
And it's been established that she regularly eats planets. Since she clearly doesn't give a crap about who lives on the planets she eats it's very easy to infer that she's killed countless lives before. Just like how we can easily guess the Diamonds from SU have committed genocide without actually seeing them do it. Rad and his race just got lucky bc of the weird burp thing they did
So yeah Cosma's a genocider