Many people have commented that the Charedi parties are only doing what everyone else does in politics. If you don't take revenge and make your enemies pay no one will take you seriously in politics and you won't succeed in getting anything for your constituents. In other words the Charedi parties are simply playing by the same rules of dirty politics as everyone else.
The answer to that is very simple. Religious Jews are supposed to be answer to a higher authority and are supposed to play by the Torah's rules and not any other rules. If you can't play by the Torah's rules in politics then don't play. Last I checked, there is no heter to violate aveiros for politics.
This is one of the reasons why the whole idea of religious parties is a terrible idea. Most Israelis see the Charedi world through the lens of the Charedi politicians and the political system which gives them a very warped view of Charedi society. IMHO we would be much better off if there were no religious parties like in the US and instead would influence the political system from within the large parties. While there is no question that the political system in the US is much more amenable to this because representatives are directly elected in districts, I still believe it could work here.
Part 1 is here