[notahacker]

Vietnam's removal of the Khmer Rouge was certainly necessary and ultimately spared a lot of people further suffering, but Vietnam's 'good guy' status in the conflict is somewhat dented by the fact that they'd provided the military support to help install the Khmer Rouge in the first place [see also a lot of other 'world policing' actions...] before the Khmer Rouge decided that Vietnam was an enemy due to a mix of ethnic and ideological hatred and being on different sides of the Sino Soviet split.

The Sino Soviet split was also a big deal for international and especially Western perceptions of Vietnamese involvement at the time: China at the time not only viewed Vietnam as an enemy but also the Khmer Rouge as an ally, and even pressured other anti-Vietnamese Cambodian exile factions to involve the defeated Khmer Rouge in their government in exile.

The regime the Vietnamese installed was less spectacularly genocidal but certainly very dubious, although it says a lot about the weirdness of Cambodian realpolitik that Hun Sen, the former Khmer Rouge cadre the Vietnamese installed as deputy PM, has held continuous office since then through years of decreasing Vietnamese influence and then a transition to notional multiparty democracy and capitalist oligarchy including coalitions with his former enemies. And that's even before we get into how many times the Cambodian king switched sides...