The 'Agnus Dei', (lamb of god), is a prayer that is recited or sung as part of Christian mass. It asks for mercy from the "lamb of god, who washes/takes away the sins of the world" - Rather pertinent for these sinners, don't you think?
The lamb of god is Jesus, the sacrificial lamb, so in this case, it might also refer to the wife, who was 'sacrificed' so that they might be together.
The lamb is also symbolic of resurrected Christ, so it could even refer to the guy's paralysed penis, which was 'resurrected'.
Lol, a lot of mangaka love to subvert Christian imagery.
oh yeah, i killed your wife and she's buried outside your home, but we'll just pretend everything is fine and look at the sunrise.