I wish to kill a service with a "sytemctl kill logstash.service" but SIGTERM is received by the service, instead of a SIGKILL. How do I send SIGKILL?

A new _logstash_ version I'm using can't stop, attempting to connect to _Elastic_ all the time, while it can't (an authentication trouble, I will resolve later). a sudo systemctl stop logstash.service isn't enough to stop it, so I've issued a sudo systemctl kill logstash.service. But it doesn't stop it neither. a sudo system status logstash.service displays:

lebihan@debian:~$ sudo systemctl kill logstash.service

lebihan@debian:~$ sudo systemctl status logstash.service
● logstash.service - logstash
     Loaded: loaded (/lib/systemd/system/logstash.service; disabled; preset: enabled)
     Active: deactivating (stop-sigterm) since Sun 2023-08-20 07:51:21 CEST; 1h 4min ago
   Main PID: 47926 (java)
      Tasks: 176 (limit: 76997)
     Memory: 1.6G
        CPU: 28min 605ms
     CGroup: /system.slice/logstash.service
             └─47926 /usr/share/logstash/jdk/bin/java -Xms1g -Xmx1g -Djava.awt.headless=true -Dfile.encoding=UTF-8 -Djruby.compile.invokedynamic=true -XX:+HeapDumpOnOutOfMemoryError -Djava.security.eg>

août 20 08:55:36 debian logstash: [2023-08-20T08:55:36,702][WARN ][logstash.outputs.elasticsearch][main] Attempted to resurrect connection to dead ES instance, but got an error {:url=>"http://l >
août 20 08:55:41 debian logstash: [2023-08-20T08:55:41,704][WARN ][logstash.outputs.elasticsearch][main] Attempted to resurrect connection to dead ES instance, but got an error {:url=>"http://l >
août 20 08:55:42 debian systemd: logstash.service: Sent signal SIGTERM to main process 47926 (java) on client request.
août 20 08:55:42 debian logstash: [2023-08-20T08:55:42,326][WARN ][logstash.runner          ] SIGTERM received. Shutting down.
août 20 08:55:46 debian logstash: [2023-08-20T08:55:46,706][WARN ][logstash.outputs.elasticsearch][main] Attempted to resurrect connection to dead ES instance, but got an error {:url=>"ht

Why isn't my systemctl kill sending a SIGKILL event, instead of a SIGTERM? And how do I send a SIGKILL to my service to force it to stop, with systemctl?