In the end, he gave his all for those he cared the most. Even then, the 'Sin' he carried with him always lingered - whether those attributed to him from birth, or those he gained while fighting for Babel - urging him to seek absolution. Some last act of kindness, to perhaps even take control of his life one last time before the death he anticipated to suffer.
A tragedy he 'found' absolution through martyrdom. Riposa in pace, sarai ricordato.
I don't think it can reach critical mass through a dead body.
You don't need explosion to infect everyone. On dying, an oripathy patient's originium lesion will destabilize rather quickly and disseminate into originium dust. Case in point: in Lingering Echoes it only took a very short time to infect Ebenholz during him caring for Kreide's body. He got infected by the cuts on his palm during the transportation.
Also, take note that this crystallization on Sin's body was not canon by in-game lore. You cannot instantly grow originium to this point by overusing your Arts; conservation of mass still applies to deaths related to oripathy---like Patriot whose body got altered by the originium mass so much he's basically more than stone than a man when we fought him. Guin must took inspiration from Faust-affected units, on which his Art utilizes fine originium particles spread by his breath (from the lesion on his throat) to travel by air and took over a target's body by activating the oripathy lesions inside infected members and control the mass, essentially creating puppets.