When we first met Sieve in the first season of Infinity Train he was just a rookie officer in Reflection Enforcement. He shied away from using a sander on the sliver they had been called in to deal with, and he showed kindness to a Prime when he had no need to.
Well, by Infinity Train’s second season Sieve had been working with Mace for a few months, and he was well on his way to becoming a killing machine. He still had a smile and a sympathetic voice for Primes, but only when he wanted something. More important, he was willing to use a sander and wanted everyone to know it.
And, of course, the season finale for The Cracked Reflection sees a bereaved Sieve turn into the killing machine he thought Mace always wanted him to be. Sometimes, Infinity Train can be cruel.