Actually he wasn't just running towards Harry, he was throwing a wand at Harry after screaming "Potter", so he would have a way to fight Voldemort. He was supposed to switch sides at the very last moment.
For the record he also took the Harry's wand out of Voldemort's hand, yelled "Potter" and threw it to him, which is how he suddenly got his wand back. Sadly this is a deleted scene (no one knows why exactly).
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