They wanted to murder Telemachus before he was full grown and a bigger problem.
They originally try to prevent Telemachus from leaving Ithaca, but he escapes. In response the suitors put twenty men on a ship and plan to ambush Telemachus on his ship.
Once Telemachus was out of the way, he would no longer be a barrier between one of them marrying Penelope.
they kill the suitors
Telemachus sailed away on swift winds, and the suitors were unable to catch him either when he left Ithaca or when he returned.
Athena
Eurymachus leads them all in their decision to ambush Telemachus.
Antinous suggests to have the suitors killed. Since Telemachus is the only suitor that keeps Antinous from Penelope, if Antinous gets rid of Telemachus and the rest of the suitors, he will be the only one left in the house and Penelope will be forced to marry Antinous.
The suitors are planning to ambush and kill Telemachus upon his return in order to secure their chances with Penelope and claim Odysseus's estate for themselves. They see Telemachus as a threat to their plans and want to eliminate him.
they kill the suitors
Yes. Telemachus fights his mother's suitors in both deed and combat.
the excuse that telemachus gives to the suitors is that he will clean the weapons
He lies to her that Telemachus is in no danger of harm from the suitors.
Telemachus sailed away on swift winds, and the suitors were unable to catch him either when he left Ithaca or when he returned.
Athena
Telemachus catches the suitors raiding the storeroom in his household.
Eurymachus leads them all in their decision to ambush Telemachus.
Odysseus tells Telemachus to hid the weapons so that the suitors can't use them. Telemachus has to say that the weapons are getting "smoke damaged" and he is afraid that the suitors will start hurting themselves with them if they have too much to drink.
Odysseus persuades Telemachus by revealing his true identity, proving he is still alive and capable of taking down the suitors. He reassures Telemachus that they have the gods on their side and that together, father and son, they can overcome the suitors' tyranny and restore justice to their home.
Antinous suggests to have the suitors killed. Since Telemachus is the only suitor that keeps Antinous from Penelope, if Antinous gets rid of Telemachus and the rest of the suitors, he will be the only one left in the house and Penelope will be forced to marry Antinous.