roy shouldnt have any problems killing at most realistic percents. once your past the point where dair > bair works, dair > neutral b will work. you can also use DED xx>, dsmash, ftilt sweetspot, and utilt sweetspot. at very high percents. uthrow works as well. it really depends on the weight class, but past combo percents, roy has plenty of single hit moves that can kill. while youre in combo percents, his kill options are pretty deadly, and you shouldnt have trouble securing the kill at all with combos around 90-110%. if you can get something near the edge, a spike kill will get them as early as 60%, with only characters like DDD, rob, and M2 surviving later than that because of their great vertical recoveries. additionally, the dair tip is surprising good at gimpimg if you can land it, which is actually not hard to do. a ledge hop dair at the ledge can lead to many early kills, regardless of which part of it you hit with.
while really good survival DI obviously extends the percentage which you can kill at, roy has plenty of mix ups available to throw that off. the simplest one i can think of is simply mixing up which way you hit them when killing with bair. dtilt or dair > bair are staple combos that are very hard to read which side roy is going to bair you on. wrong DI in the middle of small-mid stages is going to get you killed pretty early. other DI mix ups involve DED. survival DI on xx> can be mixed up with xx^<, since you know that their DI will push them behind you. another, higher level mixup is alternating between fair > bair, and fair > dair. depending on the position, you can DI a dair spike towards the stage so you can tech it; however, if you bair instead, theyve just DIed terribly at the ledge. alternatively, you can also just hit a fair tipper instead of the dair for the same DI mix up (fair tipper has smaller angle, and better KB at higher percents than the sweetspot).
overall, i dont believe that roys one of those characters that has problems killing until after 120%. hes got tons of ways to gimp, and use mix ups to get earlier kills straight up. I would try really pushing him further to land the spike more often, as well as incorporate some of the bair mix ups i mentioned.