Common misconception. Defence lawyers don't defend people because they think they are innocent. They defend them so that justice can be served. By putting up a vigorous defence, the theory is that it makes the prosecution present a case beyond all reasonable doubt.
Without a strong defence, the guilty would not be given a just decision. Without a strong defence, the innocent would never be proved as such.
Of course, mistakes are made. Police stuff up evidence. Defence lawyers/barristers stuff up evidence. But in general, without a strong defence the system is much weaker.
Give them the best defence they can get. Then, if found guilty, swing them high.