young men who have reason to fear that they will be killed in the battle may justifiably feel bitter in the thought that they have been cheated of the best thingsthat life has to offer Young men, who have a just reason to fear that they will be killed in the (upcoming) battle, may feel bitter due to the thought that they've been cheated of the best things life has to offer.

Seems ok but without more context, I can't really suggest ways to improve it. "young men who have reason to fear that they will be killed in the battle may justifiably feel bitter in the thought that they have been cheated of the best things that life has to offer." You use 'the battle' - are you talking about a specific battle or battle in a general way? For the concept of battle you don't use the article - ...in the thought...- you might want to use the preposition 'with' instead of 'in'.