Although Hezbollah is classified as a terrorist organization by many Western countries, their opposition of Israel in the most general sense is certainly justified. The Israeli occupation of Palestinian land is the result of a modern legal mandate that claims pseudo-historical evidence for its cause. Though it is certainly debatable as to whether Hezbollah's tactics are ethical, their opposition to Israel is certainly just.
Hezbollah is justified to disagree with and to make demands of Israel in terms of what it believes to be justified. Hezbollah is also justified in appealing to world courts for what it believes is justice. However, justified opposition does not mean any tactic used in opposition is therefore justified. It is important to separate the tactic from the objection. When Hezbollah eschews negotiation and good-faith diplomacy for violent tactics against civilians, its tactics are no longer justified even if its cause may be.
If one takes an objective viewpoint it is not hard to see why Palestinians would be angry at the state of Israel. They lived in this area of the world for many years before the Jewish people began to take back their homeland. Israel is not always fair and Hezbollah has a valid gripe but the violence on each side is to be abhorred.
Hezbollah is justified in its opposition of Israel because the group does not agree with the state of Israel's existence, especially considering the treatment of Arab residents in this area. Hezbollah wishes to stick up and support Palestinians, and there is no reason that Hezbollah should be discouraged in this protective endeavor. This is not an excuse for terrorism but rather regular actions of war.
No, they are not justified in the reason that they do not like the entirety of Israel. All they base their dislike on is religion, which is not justified at all. We are all entitled to worship who and how we want, and no one can say anything about it.