Number of police officers in the U.S.: 780,000.
http://en.wikipedia.org/wiki/List_of_countries_by_number_of_police_officers
Therefore, the rate of killed in the line of duty / number of officers = 0.01% (one one-hundredth of one percent).
Murder rate in the U.S. is 4.7 per 100,000, or about 0.005%, or one-half of one-hundredth of a percent.
http://en.wikipedia.org/wiki/Crime_in_the_United_States
In other words, cops are killed at double the rate of civilians.
At least according to the easily available statistics I can find.
And your point is???
According to this site 105 cops killed in line of duty in 2013.
Only 30 of those were shot by attackers. 25 died in car accidents, some of which might have been in chases, although they have a separate category for vehicular pursuit (4 dead).
10 had heart attacks. They appear to include any and all cop deaths on duty.
Police work is not, statistically, a particularly dangerous line of work. In fact, it isn’t in the top 10.
http://www.odmp.org/search/year?year=2013