Is that a power wheelchair or a manual? The power ones can go pretty fast. IIRC, some can go around 20 MPH.
I personally have no objection to shooting people who deserve it in the back. Take all the unfair advantages you can. I have no sympathy for the knife-wielder. In my book, pulling a knife on cops (or me) and threatening to do harm means that you are desirous of endangering life, and ought to be shot repeatedly, your method of motion be damned. We've got enough stupid violent people in this country.
Could the cops have subdued the guy without putting holes in him? Sure. Pepper spray, tasers, batons. I figure that cops are poorly trained on baton use these days, since massive bruises look so bad to a jury.