Dear Riku,
Please don't think of what is happening to you as "stealing" but instead think of it as securing. A kill that is "secured" keeps a foe out of the game for X number of seconds and the number scales depending on how long the game has been in play. This means, near the end of the game, any killed foe will be out of the game for anywhere to 30+ seconds which can be a complete game-changer or game-finisher. The fact you and your friend do not anger over what you described as "stealing" is something I'd consider to be very positive and hopefully you'll be able to start "securing" kills for your team as your playing skills progress.
Best regards,
Mr. Beane.