One of the stronger factors influencing the frequency of wars is the ratio of young men to older men. Life extension would change that ratio to imply fewer wars. See http://earthops.org/immigration/Mesquida_Wiener99.pdf.

Stable regimes seem to have less need for oppression than unstable ones. So while I see some risk that mild oppression will be more common with life extension, I find it hard to see how that would increase existential risks.