"Твари" or "мрази" are the best words I can think of. Neither of them are truly obscene, though. The genitalia-derived obscenities offered in other answers have something comical about them, not something you would seriously call a strong and dangerous enemy who has glassed so many planets (unless you want to portray the situation with a degree of humor). When you call a person something like "уёбок" or "долбоёб", the nuance is that he is a somewhat inept person, doing his bad things at least partially out of stupidity.
Can't refrain from offering my very valuable opinion on the matter in general. I am a native speaker of Russian with semi-decent English. I also speak enough Japanese to get by. I like cursing (yes, in all three languages) and often do, but can't remember ever using a curse from a language foreign to the one I am speaking at the moment. That would just feel sooo awkward. The grammar is different, the phrase composition is usually different enough so that there's simply no good place to insert a foreign curse into. The way you [are trying to] pronounce "corresponding" sounds in different languages is also different, should I pronounce it like I would in English, e. g. "moo duck", or the plain Russian "мудак" will do? Switching takes enough time and mental effort to kick you out of the flow. The closest thing to your example I can imaging myself saying is something like: "What are you doing, you... you... [struggling to remember the word "faggot" for a second and failing]... пидор!"
And no, "Clockwork Orange" is not a good counter-example, what's described there is a sub-cultural phenomenon of taking a very limited subset of foreign words and adapting them to English grammar and phonetics.