I didnt see her slapping his arm away in the video. If she did, then she (now) knows better than to slap a police officer.
At around 2:32, the officer approaches the lady, says something to her (can't tell what is said) and he goes to grab her arm as if to place her under arrest. Lady then becomes non complaint at that time, forcefully pulls away from the officer, turn her back and then begins walking away. We don't know what happened prior to the camera footage and we don't know what he was placing the lady under arrest for. But based on what I saw in the footage, it appears an officer goes to grab a lady's arm, she actively resist arrest, and then gets tasered for her trouble.
In the police use of force matrix down here (meaning all of Florida. I was a deputy for 10 years down here), the lady would be offering whats called "active resistance" and it would be 100% justified in tasering her. Active resistance is when one braces or tenses ones arms to prevent handcuffing, pulling away from an officer, walking/running away from and officer, etc. People have to realize that police use of force doesn't look pretty. It often looks way worse than it is.
What I believe happened is this. The lady was running her mouth and generally ticking off the police. They decided they had enough of it and decided to charge her with obstruction. Once the officer made up his mind that he was arresting her, he moved in for the arrest. She offered some resistance, and he popped her with his taser at the first opportunity. Was it justified? Yes. Did it look bad? Yes. Should he have used more verbal commands and use the available backup officer to place her into custody? Probably.
By the way, put yourself in the officers shoes. During the earlier parts of my career, I neglected to use a taser when I was justified to and I was stabbed with a needle and had to undergo two years of HIV/Hepatitis testing, which placed considerable strain on me and my marriage. Luckily, I didn't catch any diseases. About a year later, I again neglected to use a taser when justified and decided to go hands on with a robbery suspect. It turned into a violent knockout dragout style fight and I was fighting for my life. The suspect busted my taser in half in the middle of the fight and he also busted my knee up. I have permanent knee damage and will have to see a knee specialist about every 5 years for the rest of my life. Later in my career, every time I was 100% justified in tasering a person, they got tasered immediately, up front, to take the fight out of them. I wasn't risking getting stabbed again or getting my knee busted up again. Sure I could have went with other methods, but the taser simply works most of the time and is proven to reduce injuries to officers and arrestees.