You really couldn’t blame them for trying. Having crucified Jesus, the priests and leaders of the day really wanted to make sure that there would be no further “issues.”
Matthew 27:62-64: “Now the next day, that followed the day of the preparation, the chief priests and Pharisees came together unto Pilate, Saying, Sir, we remember that that deceiver said, while he was yet alive, After three days I will rise again. Command therefore that the sepulchre be made sure until the third day, lest his disciples come by night, and steal him away, and say unto the people, He is risen from the dead: so the last error shall be worse than the first.”
That Jesus was dead was a verified certainty. The crucifixion had done the job, and the spear rammed into his side and piercing his heart had served as a double confirmation. The soldiers had made sure he was dead. The chief priests and Pharisees testified to his death with the words “While he was [past tense] alive.” And yet, knowing that Jesus had promised to rise again on the third day, they quickly approached Pilate and asked for the means necessary to ensure that his body stayed right where it was at.
And when they did, they used a pretty interesting word. When they said, “Command therefore that the sepulchre be made sure until the third day,” that word “sure” is from the Greek root word asphaltos. We get our English word asphalt from it. They were basically asking that the tomb be as firmly sealed as if it were paved over. They were not interested in taking any chances whatsoever.
Their stated fear was that the disciples would come by night and steal him away and thus “stage a resurrection.” Obviously, that was a ridiculous fear. There were only 11 regular followers of Christ left, and all of them were scattered in terror. Only one of them even made it as far as the cross to see his Lord die. And even at their very best while Christ was still alive, all of them were just common laborers, and all of them were erratic and unreliable.