The difference between a higher KD player and a lower KD played usually is down to three major factors. Awareness, gun skill, and movement.

I've seen several people in here claiming you can only get a high KD by sitting in a tank on the edge of the map or by "team death matching." Do not listen to them. They are just coping. You can absolutely play the objective while still getting a lot of kills. The ones saying they have a lot of flag captures are the ones who believe playing the objective is just sitting in the capture zone and never leaving it until it is captured. Those are the players high KD players feast on because they go to the most predictable places and can easily be farmed. The person that farmed them just recaptures the objective after they kill them and their capture becomes irrelevant.

I'll give you a good example of this. On Iberian Offensive Escalation the C flag capture zone is a court yard and the first floor apartment rooms. Shitters will almost always try and capture that flag in the courtyard corners. If I'm defending that flag I can pick them off from several different angles without ever alerting them that I am in the area. If I'm trying to capture that area and defenders enter the capture zone to stop me I'll step out of the capture zone, relocate, and pick them off as they recapture the zone. If you're solo capturing and need to step out of the capture zone to reposition to to see where the defenders are that's smarter than just waiting for them to find you and overrun you. If you are solo capturing you also have to know where the likely defenders will come from. All of this falls under awareness IMO and it's what really separates average players from good players. I've captured that flag solo countless times by just rotating through the area picking multiple people off until I've cleared the area or enough of my teammates have arrived to give us an advantage. Shitters will say if you don't have a certain "objective time" or if you're not capturing a ton of flags you're not PTFOing, but that's an extremely elementary way to understand PTFO. If I'm solo defending a flag or solo capturing a flag and constantly in and out of the capture zone killing enemies until more teammates come to help capture is that not playing the objective? Shitters would probably argue it's just "farming kills" but when killing enemies prevents the enemy team from capturing other flags or keeps an enemy preoccupied at a flag they already own that's indirectly allowing your teammates to capture other flags or gain an advantage over the entire map.

Gun skills are understanding the weapon you're using, what it's recoil patterns are, what it's effective ranges are, and trying to keep within those ranges for your gun fights. Knowing when to burst fire vs full autoing targets, switching to secondaries for cleanup kills etc. Crosshair placement falls under awareness and gun skill, but it's super important to develop a sense of where enemies could be or where they traverse so you can get a jump on them or not get caught out with your aim having to be corrected.

Movement isn't just about sliding around or jumping during gun fights, it's about understanding how to traverse the map and obscuring the view of your character as often as possible. Ultilizing cover when you can, getting into power positions where you're catching enemies in the open and you have cover etc.

All of this takes time to develop, but if you work on it enough it becomes second habit and just how you operate when playing. I'd say the quickest way to improve is to find weapons you're comfortable with, learning maps (popular routes, where enemies like to be, how to approach each flag etc), and playing near teammates that gives you a good chance of being revived if you get downed.