Why was Leo so mean to Chris?.

Chris was an ass to Leo as soon as he sent him to Valhalla where I'm sure he hoped Leo would die - the best reason for Leo to be an ass to Chris before he knew who Chris was - a stupid reason to not keeping being an ass to the brat.

Chris had NO reason to be an ass to Leo. No matter what his father did or didn't do in the future (and the only reason he became an Elder was that Chris made him one, so I don't think that happened in the future - I think that was one of his many lies like telling Piper that there's no such thing as a family in the future or that Paige died but also gave him money when the tour guide made it clear that Paige lasted quite a bit beyond 'Oh, My Goddess!' in order for the Charmed Ones to have had that many vanquishes before being vanquished themselves, which seems to imply that they died together, meaning him talking about Piper dying when he was 14 was also a lie.) he hadn't done it yet - and thanks to the way Chris treated him, Chris was almost not conceived - pretty stupid.

No, the much bigger ass was Chris, one of the worst characters on Charmed, especially because he did absolutely nothing to prevent Wyatt from turning evil as proved by the fact that Evil Wyatt showed up in Season Seven. Should've had his ass sent back to the future as soon as the sisters knew about Wyatt.