most likely possibility to me has always been that he did kill himself - the doj gave him the means to commit suicide and he accepted; this letter and perhaps others were written between the acceptance and the act.

when you're on suicide watch in federal prison, if they don't want you to die, there's no way you're going to die short of biting your own tongue off and choking on the blood. so rather than see this as 'he was planning suicide', see it as 'he was secretly offered death as a plea deal and he accepted'.