You read that right. She didn't even vote against the Magnitsky Act itself. She took federal office in 2013 the year after the 2012 Magnitsky Act was passed. The text of the 2012 Magnitsky Act (after a global expansion), was put into the 2016 National Defense Authorization Act for Fiscal Year 2017 (the must-pass bill encompassing most military actions). When you have an all-encompassing bill like the National Defense Authorization Act, you can support some parts of it and not others and consequently vote against it- either in protest or because you'd want it to go through and be amended. Bernie Sanders voted against the 2012 version of the bill because he wanted to improve it, too.

The reasons for voting against the bill varied- for instance the ACLU recommended against the bill because it kept open the prison facilities at Guantanamo Bay, which they stood staunchly against. Tulsi Gabbard released a statement explaining her reasons for voting against the bill, which you can read here. She voted against the bill because it would authorize giving the terrorist organizations within the Free Syrian Army (one of the factions we supported against Assad), sophisticated anti-aircraft weapons, MANPADS, which are capable of destroying U.S. fighter jets. She viewed our alliances with this faction that openly had jihadist terrorists among their ranks as fragile and not to be trusted, and she was vindicated when the United States stopped supporting them, along with all other NATO nations (sans Turkey) in 2017.

I learned in 8th Grade Civics class that you should never trust a smear commercial saying "This incumbent voted against the Aid-For-Children-With-Cancer Bill AND the Support-Our-Veteran-Amputees Bill" because those are always intended to mislead you into thinking they opposed a bipartisan common sense bill instead of wanting to improve it. I'm upset to hear so few have learned this.