User blog comment:B214/Volume 22 spoilers/@comment-201.215.165.7-20161224184456/@comment-10330822-20161225114239

Based on Indra's personality, it is also possible that Vvritra was not as "evil" as the story says, and Indra just used that as an excuse to kill him. After all, the Asuras seem cool, and Indra warred against them because they were "evil". My guess is that Indra is just self-righteous, and so he views himself has the ultimate good and anything that fights against him or stand against him "evil".

Vvritra was probably broken into several different gears because of all his abilities, they couldn't be confined to one Sacred Gear. However, since they are all parts of the same dragon, they can be collected and used by a single individual to gain more and more of Vvitra's original power.