What the four traps all have in common is that the person setting the trap feels justified in his actions, and therefore shows little sympathy for the person he is trying to trap.  Polonious thinks it is his responsibility as a father to ensure that his son is staying out of trouble.  Even Reynaldo questions the wisdom of making suggestions about Laertes' bad behavior just to see how people react, but Polonius thinks that the plan will reveal the truth of Laertes' behavior, never mind that this could actually soil his reputation.

Claudius, with Gertrude's approval, brings Hamlet's friends in.  Gerturude is legitimately concerned for her son's welfare, and Claudius is concerned about his own position which he has already killed for, so I don't think he is too concerned about what becomes of Hamlet. 

Polonius has just shown us a lack of care for his children, so using Ophelia to get to Hamlet is not a big leap.  He has already told Ophelia to stay away from Hamlet, so probably assumes that a "accidental" meeting isn't going to hurt anyone.

Hamlet certainly feels justified in trying to trap his father's alleged killer -- it is a necessary means to an end.  He feels he must have proof that the ghost was telling the truth in order to feel justified in his revenge.  If Claudius is NOT guilty, then there will be no "conscience to be caught" and no one will be the wiser.

In Scene one, Polonius sets a trap to attempt to discover what Laertes is up to -- is he falling by the wayside by gambling and visiting brothels?  He counsels Reynaldo to act as a sort of spy, asking misleading things of those around Laertes to try to get the truth by way of deception.

In Scene two, Claudius has sent for Rosencrantz and Guildenstern, friends of Hamlet's, to come to court in Denmark to serve in a similar scheme to the one set up by Polonius in Scene one.  They are to hang out with Hamlet and try to uncover why he is suddenly behaving so crazily.  Claudius intends to trap Hamlet into confessing the motive for his behaviour.

Also in Scene two, Polonius schemes with Claudius and Gertrude.  He says:

You know sometimes [Hamlet] walks four hours together

Here in the lobby....

At such time I'll loose my daughter to him.

Be you and I behind an arras then,

Mark the encounter.  If he love her not,

And be not from his reason fall'n thereon,

Let me be no assistant for a state.

The final trap is  the one that Hamlet sets to "catch the conscience of the King."

He says:

I'll have these players

Play something like the murder of my father

Before mine uncle.  I'll observe his looks;

...If a do blench,

I know my course.

And so, in Act Two, are many traps set in place to discover information by covert means.