>Police officers are almost entirely decent people . . .
Between 20% and 40% abuse their families. Does an entirely decent person abuse his family?
I'm not aware of a higher family abuse rate among EMTs, and a quick Google search produced nothing, so I'd appreciate any sources. Your reasoning makes sense, but I can just as easily say that violent people are more inclined to be police officers and soldiers - for obvious reasons. I'd guess it's some of both (some high stress and the attraction of the occupation to violent people).
Yes police officers have higher rates than the average, but you know who has higher than even those wife beating cops? Men that work in "demeaning jobs" or professions that are attributed traditionally to women like clerks, secretaries, teacher/classroom aides and maids/helpers.
The vast majority of those articles boil down to a handful of studies the one linked below is probably the most commonly referenced.
http://onlinelibrary.wiley.com/doi/10.1111/j.1741-3737.2002....
And yes most police officers are decent and professional people, but there isn't such thing as being universally decent and any person is capable of the utmost indecency.
And you know what they say about so many intimate partner abuse cases? he/she was the nicest person in the world I would never think they would be capable of it....
People that abuse their partner more often than not do it as a release mechanism rather than their normal pattern of behaviour.
What is that based on?
Take a wild guess on what those have in common with police officers.
People that are constantly exposed to highly stressful situations tend to develop psychological problems.