Lamenters are a wild card as the whipping horse of Chapters. They have a history of poor judgement and are likely genetically corrupted. I'm going to refer to a book that indicates the only nemesis of the former legion is Ka'Bandha (recurring as often as Cobra Commander) and whose only salvation is jump packs and Sanguinor, but check page 17 and 54 of the Blood Angels Codex for some new freshly printed tidbits on the Lamenters. Lamentors? Their medical claims are about as credible as a Sunday morning miracle cure advert and they seem hesitant to be seen by any sapient creature except those on the other end of their bolters.
At any rate, there are many factors contributing to decisions made during a crisis. There were no doubt communications coming from both sides of the conflict, the Mantis Warriors believed one side over the other and acted. Failure to act is a failure of character for any soldier of the Imperium, disgraceful for a Space Marine (a breed that sees valor rather than folly in trying to punch to death a greater deamon).
