Judas approaches Jesus with a greeting and a kiss. To modern readers the action may already seem intimate, but in the ancient Mediterranean world a kiss could function as a conventional greeting among relatives, friends, and respected associates.
The betrayal therefore corrupts a gesture normally associated with relationship and loyalty.
Kissing as greeting appears in Jewish and wider Mediterranean sources and could communicate affection, respect, or welcome. Judas arranges a sign that will identify Jesus to the arresting party, and the chosen sign allows him to approach Jesus as a disciple might normally approach a teacher.
Mark also preserves the address 'Rabbi,' intensifying the outward performance of discipleship at the moment Judas hands Jesus over.
The cultural meaning makes the betrayal more than a covert identification technique. A gesture of fellowship becomes the instrument by which fellowship is violated.
Jesus' question in Luke—whether Judas betrays the Son of Man with a kiss—draws attention to precisely that contradiction. The external signs of loyalty can be present while the act itself is treacherous.