Why did Dracula do that to Mina?

Dracula saw an opportunity to put her under his thrall and took it. He enjoyed doing it right in front of everyone, without them realizing what was happening -- only Lucy got an inkling of what he was doing. As a bonus, it put him back in the center of attention and caused Lucy and Jonathan to discontinue their dancing. It also took focus off of the Romanian journal they were discussing, that no doubt went on at length about a vampire on board ship.

There's also the point that, despite having "saved" him, Mina was quite... lacking in Dracula's eyes, too easy a target for his predatory nature. He wanted more of a challenge than a sweet but constantly ill girl, and found in her friend Lucy.

He said it outright at the dinner party, though obviously nobody understood his true meaning at the time. "I despise women with no life in them... no blood."

There's also the rather likely possibility that there was an element of revenge to Mina's death. He was, of course, already going to kill her, but the timing was... interesting. Watching Lucy and Jonathan from his unseen perch on the roof in the previous scene, it's pretty clear he's seething with jealousy.

So he turned Mina into that wreck of a creature, obviously having finer plans for Lucy once she turned.

Because he was a monster. I´m happy he burned in the sun!

“Look, you don’t really think that I could be in love with a rotten little tramp like you, do you?”