A 3.5% rate suggests that he had more than 20 chances to throw out a base runner and only got one. Since I'm pretty sure I remember him getting more than one guy who tried to stretch a double into a triple, my guess is that they're defining kill rate as 'Got an assist on a ball that ended up in his hands as something other than a put out.'
That would include people who never tried it because they knew his arm strength. I could be wrong and I could just be mind facting it. But I really question him only getting 3.5% of every single legit chance. That'd be a ton of offline throws, errors, etc.
**edit: so I looked it up. MLB avg for Left Field? 2.2% Kill rate. MLB avg Right Field? 2.3% Kill rate. Bryce in Right: 4.3% career.
I don't think those count as bad stats or reasons to say Bryce sucks.