I'd have to disagree with that. Vader was obviously pissed off by her attempt to attack him, and he is a darksider whose power is fuelled by passion and emotion and has a reputation for killing people who annoy him. From this point of view, killing Juno is perfectly in character for him.
As for his plan, it doesn't matter to him if Starkiller submits to him or not. If his plan was simply to make Starkiller submit by threatening Juno, then he wouldn't have tried so hard to make him give up his attachment to her in the first place.
Way I see it, his plan was to destroy the Rebellion by sending Starkiller back to them, then having him betray them. Only he wouldn't, because Juno's a Rebel now and he wouldn't betray the woman he loves. So Vader tried to make him let go of this attachment, which didn't work out all that great.
With Starkiller gone, Vader switched to plan B. Send out Boba Fett, bring Juno to Kamino, then try to coerce Starkiller into cooperation by threatening to kill her. That didn't go down all that well either, since Vader struck her down in a fit of rage.
But the beautiful thing is he can absolutely afford to do that, because he's got plan C. He has the Dark Clone. He tried to turn Starkiller because, well, two loyal and obedient demigods of mayhem and destruction are better than one, but ultimately it makes no difference if Starkiller joins Vader or not. Vader can simply kill him at any point and send out the Dark Clone in his place, which is what he was preparing to do at the beginning when he was getting ready to stab Starkiller in the back again.
It's essentially backup plans within backup plans within backup plans. And the best thing is that it worked perfectly even in the LS ending. In the DS ending the Dark Clone reveals himself because Vader is about to get killed. In the LS ending he has no reason to do that, but that doesn't mean he's not there. He simply remained invisible and in all likelihood stowed away on board the Rogue Shadow, en route to the secret Rebel base that Vader probably didn't even know existed. The LS ending works out better for Vader than the DS ending - not only has he learned of the existence of a hidden Rebel base, he's managed to trick the Rebels into taking him there along with a loyal clone of his superpowerful apprentice, whose very existence the Rebels have no idea about. Cunning enough for you?