Generally speaking most Buddhists are vegetarian out of trying to hold compassion for all living things. However, in reality it's only generally enforced amongst the monastic order, who are expected to hold a higher standard of spiritual development - especially those who have taken Bodhisattva vows (Mahayana/Tantrayana paths). Even then it's not necessarily true all the time. For example, monks and nuns will eat whatever is given to them during alms collection (food collection/ritualised begging).
Also amongst the laity meat eating is prevalent, and is a decision for the individual. Often the devout in traditionally Buddhist Eastern countries will not actually kill animals but rely on neighbours of other faiths to sell them slaughtered meat (rather hypocritical in my eyes, but whatever). This is certainly the case in Tibet. While not killing or eating meat could be considered right action (a tenet of the Noble Eight Fold Path), it's not a 'commandment' of Buddhism. In fact Buddhism is, in itself, a philosophical framework upon which an individual is encouraged to develop their own spiritual nature guided by the experience of those gone before. There are no commandments or judgements to be meted out. It's a matter for the individual.
All living beings are entitled to compassion, and as a Buddhist you would (try to) understand that any negative deed is not only incurring karma for the perpetrator, but also a manifesting result of previous accrued karma. Thus, one should be compassionate towards them. Further, all beings are Buddha and possess a nature stemming from the Dharmakaya (Buddha consciousness). As such despite the grossly manifest appearance of delusion and negative deeds, ultimately they consist of emptiness (shunyata) and are pure in nature. Depends on which level you want to look at things (Hinayana, Mahayana, Tantrayana).
A Zen (especially Sōtō schools) practitioner would caution that all beings are Buddhas, and that some Buddhas teach us by example how to behave, and others teach us by example how not to behave. We are each personally responsible for our own behaviour/karma, but that doesn't mean we have to be discompassionate to other beings based on their level of spiritual ability, delusion or action.
In fact HH The Dalai Lama would caution you to be even more loving and compassionate towards someone acting in a way you perceive to be cruel or unethical.
That's a summary, I'm too tired to type an elaborate reply citing sutras and lectures. Sorry.
Source: Twenty three years studying and practicing Buddhism under Dr Akong Tulku Rinpoché (Samyé Ling Tibetan Centre, Dumfries) and Sōtō Zen (Throssel Hole Buddhist Abbey).