ἀλλὰ ταῦτα οὗτος μὲν ἴσως οἴεται καὶ ἄλλος τίς που μεγάλα κακά,
ἐγὼ δ᾽ οὐκ οἴομαι, ἀλλὰ πολὺ μᾶλλον ποιεῖν ἃ οὑτοσὶ νῦν ποιεῖ,
ἄνδρα ἀδίκως ἐπιχειρεῖν ἀποκτεινύναι.
I know that μᾶλλον can be accompanied by the positive, as here with μεγάλα κακά.
But shouldn't there be ἢ, than, before the thing it is compared to?
Perhaps there is an Attic crasis for ἢ ἃ = ἃ, or maybe it is implied and omitted.
Another question: ἄλλος τις is the subject of ποιεῖν with οἴεται implied, right?
a) the "μᾶλλον" construction: It's the exact same as "While it's bad to steal from someone, it's much worse to shoot him"
In both cases a "than doing A" is of course inferred. In our particular case the whole sentence could be (as usual on the fly) "[οίομαι] α ουτοσί νυν ποιεί [είναι] πολύ μάλλον κακά ή τούτων" or something like that. Or, in English "I don't think those are great evils but I think that what he is now doing is much greater evil than those". There's also the possibility it's a case that doesn't translate all that well of "I rather think" but, apart from coming up with the full-fledged sentence there's no real difference.
If "ἃ" stood for " ἢ" the comparison would be reversed wouldn't it?
b) "ἀλλὰ ταῦτα οὗτος μὲν ἴσως οἴεται καὶ ἄλλος τίς που μεγάλα κακά"
Actually "άλλος" is the subject of "οίεται". I, personally, think that the infinitive missing is "είναι"
Τούτος και άλλος τις (που)
ίσως οίεται ταυτα μεγάλα κακά.
"[..] he and someone else somewhere may think that these are great evils but I don't think so. What he's now doing, trying to put a man to death unjustly, is a much greater evil"