That is exactly why the American Left is in love with Islam. Although their goals are polar opposites, their mindset and tactics are the same.
That is, that terrorism and innocent deaths are part of the price to be paid for victory. It is a victory at any cost mindset they both share, just ask Bill Ayers, Alinsky, and the Weathermn.
And most of all, the ends always justifies the means with both the American Left and the Islamic terrorists. They share this central tenant in common.
You may wish to rethink your argument, at least in part.
Both radical Islam and the collectivist Left tend to rationalize their worst behavior in service to a "greater" good: In the case of Islam, it is the duty to convert or kill all infidels.
Whereas the Left rationalizes its
behavior on... a duty to kill or convert all infidels. Just kidding. For now.
Today's "Netroots" Left is characterized by both
a Utopian and a Manichean (morally dualistic) mindset that insists not only in the perfectibility of mankind, but increasingly, in its division between compassionate Progressives and dangerous Reactionaries.
The perceived duty of both is to remake the world, and those who stubbornly refuse to be remade and "corrected" will not be treated kindly. Their goals therefore, are not "polar opposites", as you suggest, but very much the same.
In the service of such ideologies, it is not difficult to see how any means might be justified to serve their ends, no matter how lacking they might be in human virtue or common decency.
The difference between radical Islam and our own collectivist Left lies in their respective perceptions of reality: for one group, truth is that which respects and serves the tenets of their time-bound faith; all else is false and unholy. For Left/Progressives, truth is entirely subjective, unknowable, and ultimately subservient to Justice, as defined by an equal division of things and attributes ("distributive justice"), supported by a belief in economic determinism, and enforced by government fiat.
One group worships Allah and his earthly emissaries; the other, the State. Both groups despise individualism and the idea of natural rights.
Given enough power and resources, both will try to kill you if you get in their way.