I think it's weird that Kalec wants to use the Heart of Zin-Ashari. He tells us that the Heart was once used to control the Well of Eternity. I think it's safe to assume that that's what Kalec is planning to use the Heart for himself. He uses us and the Moon Guard to empower the Heart and then when we're ready we get a new artifact appearance but Kalec takes the Heart with himself. The Blue Dragonflight was always against the magic but Kalec didn't seem to care. He was harboring these feelings all these years since the lost of Aviana because they all were tricked by Alextraza's consort. The sunwell was saved, but the love of his life died in the process.
Then he met Jaina. She leaves Dalaran and I don't think Kalec would just let the Kirin Tor to treat his girlfriend like that. They deviced a plan: use the Heart of Zin-Ashari to control the Well of Eternity and wreak vengeance.
Kalec was nevert to be trusted. Even Khadgar hinted at it.