For #1, the
Fire ClawsRelictors spring to mind. In fact, they may well be the perfect example for such a development. Not anymore, however, since the Chapter was chastised, so they are more of an object lesson as to what could happen to other Chapters, rather than a Chapter that would still do such things "now".
#2 is more tricky, simply because most armies' fluff focuses on heroic exploits and epic victories rather than such dark hours, and this "romanticism" runs especially strong in most material about the Astartes. There is precious little background on Chapters with such a critical flaw that did
not outright turn them into Renegades. However, in the
CSM 'dex there was this bit about a number of Space Wolves on the strike cruiser "Wolf of Fenris" turning their guns on their former brethren and joining up with Huron's Red Corsairs as their ship was being boarded, apparently preferring switching their loyalties to Chaos rather than being slaughtered in a battle whose outcome was already decided. Assuming that the troops stationed on this ship were an average selection from amongst the Chapter, this could reveal a dangerous streak amongst the
SW, which however fits to the individualism propagated by the Chapter's culture. Other than that ... I can't think of any obvious betrayals - unless we expand this definition to also include Chapters at risk of falling victim to a dangerous mutation of their geneseed, such as the Flesh Tearers, or at least some of the Chapters created by Inquisitorial mandate during the ill-fated
Cursed Founding.