Mercy and Justice are not mutually exclusive. God is merciful AND just.
We can and should pray for Ted Kennedy's immortal soul. I pray to God he will be merciful. I don't want anyone to go to Hell but that is a reality for some. Hell is real and you can end up there.
We should not canonize notorious sinners nor afford them public spectacles. It scandalizes the Church, weakens believers faith among other things. That is not just nor merciful.
I didn't read the word "mercy" in any of his public pronouncements. Maybe I missed it. If he wanted to use the Kennedy funeral as a "teaching moment" he could have recounted the reasons for mercy and urge prayers for the repose of his soul. Instead we see attribution to what was a scandalous life by the eulogies presented in Kennedy's send-off.