To "involve" someone or something means include them as a necessary part of the result or rather, they were part of the situation somehow and are key to solving it. To "need" someone/something, however, means to require something/someone because they are (or could be) important to the situation, but not necessarily part of the situation.

I think Saul's answer is correct, but it can profit from some illustration, because 'involve' has two meanings, closely related but not identical. The first definition is the one Saul gave, involve means to include as a necessary part or result. Say you want to make a cake. I am a terrible cook, but I think all cakes are made with eggs (otherwise why call them 蛋糕？？）In order to make a cake you need to break the eggs. You cannot make a cake without breaking eggs, so making a cake involves breaking eggs. Now you may need to buy eggs in order to make your cake. But making a cake does not involve buying eggs, because you might get them from someone else or maybe you have some chickens in your backyard and can just go there to get some eggs. So to say that you need to buy eggs in order to make the cake says something about you, namely that you cannot get eggs otherwise, and not about the cake-making process. The second meaning of involve is to cause someone to participate in some activity or situation. So you can say "project A involved all the people in our department" , to say this is not to say that everyone was necessary, it only means that every one took part in the project in one way or another. Maybe it could have been completed all the same even if half of the people had not taken part. But to say "Project A needed all the people in our department" is to say that every single person was necessary, and if just one had not helped the project would not have been possible. Sorry to bring back an old topic, but I found this interesting, because involve and need overlap, but this overlapping is different according to which definition of 'involve' you're using.

thankyou！

Involve means you are included in a situation. "I was involved in a car crash." "I was involved in a crime." This means that you participated. Need means that you want something. "I need money." "I need to go to the toilet." This means that you desire something.

When the case "involves" many innocent people, it means that the innocent people are already a part of the case. They are in the situation. When the case "needs" many innocent people, it means that in order to continue the case, innocent people are needed. They are not in the situation, but now it is necessary for them to join the situation.