No surprise perhaps to find me voting No as to their nature and by default being harmful (all depends upon the doctrines and such), but where the opportunity is obviously there, even with the evidence that many don't need any other reasons to do harm. Some examples as to why we might cause harm:
we might have harmful beliefs (even if we don't regard/recognise them as such) and these perhaps being religious ones;
we might react to criticisms or ridicule/slating of the above, e.g., perhaps citing blasphemy;
we might feel a strong allegiance to some group and hence go along with whatever the group decides or promotes;
we might be easily persuaded (perhaps by others or what we regard as evidence);
we might lack understanding and/or knowledge (regarding others, in particular areas, or in general);
we might feel a need to inflict harm/pain on others or to control them;
we might be jealous of what others have or feel they are not entitled to such;
we might fear or feel threatened by others or what they might do;
we might feel our wants or needs trump those of others, and hence disregard theirs;
we might not realise that we are harming others;
we might harm some (children, for example) because we think it necessary and for their benefit;
we might lack morality or perhaps have our own and disregard that of others or any societal norms;
we might view others as less 'worthy' than us;
we might be damaged in some way (born as, or possibly being from our parents, peers, or societal treatment/attitudes);
we might lack empathy for others (in general or at the time for various reasons);
we might harm others (those we love) to hurt ourselves (a form of self-sabotage or self-punishment);
we might simply enjoy harming others, and get habituated to such through feedback;
we might harm others in order to protect those we love, or do so in anticipation of such;
we might harm some - as in examples of mercy killing - to end a painful existence;
we may have been harmed ourselves and feel the need for revenge or to retaliate;
we might 'Other' some - simply disowning them as not being human (paedophiles, for example);
we often harm others by accident of course, and sometimes when we don't take enough care;
we might be affected by drugs or alcohol so as to be less in control;
we might just lose our temper or control of ourselves for a variety of reasons;
Our threshold for harming others might be lowered by circumstances, e.g., due to addictions or obsessions;
we might harm others in order to harm those we cannot harm, so these essentially being proxies;
we might harm others whilst defending ourselves, and this often being appropriate;
or any combination of these and to more or lesser extent.
You've noticed a gap in the list? Although debatable, many of the things that might cause harm to others could be associated with religious beliefs (the top section), even if many of these might also apply for other ideologies, but I think this perhaps does show that a religious belief could cause many harms as well as any good they do.
And this latter is my position, but not that easily demonstrated.