Because of human nature and its offspring mores in current US society, religion has a protected, elevated status as a motivating factor in behavior, and is given a latitude far beyond any other influence. Because liberal believers dare not or cannot challenge this, they indeed have some responsibility for the nefarious actions it permits in their less liberal co-believers. Human nature per se has no such status and no such willing defenders among the liberals who understand it.
First, in terms of elevated status in motivating factors, religion is not the only game in the US. Capitalism is equally, if not more exalted. Individual freedom is the paramount value, often to absurd, anti-social, and destructive excesses in it's name, looking at libertarians, survivalists, tea-partyers, and any of the extreme small-government crowd. The right to bear arms as percieved by many Americans as absolute, and excuses all kinds of bad behavior. Look at the "Stand Your Ground" laws.
Secondly, a liberal believer has a different belief than a conservative believer, and that conservative is no more likely to listen to a liberal believer than a liberal atheist. A conservative Christian would not even probably consider the liberals Christian at all, because they define membership in the group differently.
Third, liberal Christian protests are often not covered by the press, or if they are, they are not prominately featured, as they are simply not controversial enough to be interesting news. We have put links in this forum to actions taken and groups protesting, but that has done nothing to prevent this meme from circulating over and over again in this forum that liberal believers are not doing anything. It is utterly false, but it persists.
We are each individual actors; we have no responsibility for the behavior of others, nor could we control them even if we wanted to. Liberal Christians have no greater responsiblity for the behavior of conservative Christians than atheists do. Liberal Christians have no greater control over conservative Christians than atheists do. That is just the way it is. It is neither our responsibility or within our power.