How much does it actually give per kill?
It gives melee combat strenght x 0.5. Thus makes it better to kill melee than ranged/siege, which have a low melee strenght.
Again, it can be ok, but you really have to go after it, which may not be worth it for city placement/opportunity cost/you don't have an army yet/hammer and maintenance cost reasons. If you are gonna give it a go, put on raging babas or:
1)
Find your target. Look for the civ that has built the most units (see military adviser), and try to go for civs that make melee (especially mounted) heavy armies (e.g. not babylon, who will spam bowmen. Generally those with melee UU's later will have a melee bias early too). Avoid early UU's that give non cs bonuses (jaguar bad, hoplite good) but go after those with high cs like the legion.
2)
Piss them off. Inducing wars can be fun if done correctly, even warmongers need to know some diplo. hopefully they'll send their army over to your cities for the bonus.
3)
Plant a city (with your army nearby) RIGHT next to them. Try to find a spot where the land between you and them is flat and easy to kill units on. This means if all else fails you can DoW and still get the bonus, itmight also induce the DoW. Try not to take cities cos they'll not have the pantheon and their units will try to take it back. You'll have to kill them for 0 faith. Don't forget to take your army to the new city, else it'll die
4)
Don't let them be at war with anyone else, cos they'll lose units that you can kill. Tough to do but you can pay people to make peace etc.
5)
Focus on killing units outright. Seems obvious but 2 nearly dead units give 0 faith. Also target melee, especially mounted.
6)
Piss off everyone else (neighbour only
). You might get an extra DoW when you run out of units to kill from the first war.
The main problem is not in fighting them, but in getting the bonus when you do, you can't just dow someone and march over there, so it takes a little skill. It's generally not as powerful as some others (e.g. one with nature with spain and GBR, immedaite +16 faith!), but if your other options are tough, it can be made to work, and it feels very satistfying when it does. Plusif you'resettingupfo domination, you'llalready have the most important part (an army) and it's a greatway to benefit while you work up experience early.
for killing a barbarian brute. With God of War, you get 4 
for killing a barbarian brute (or AI civ warrior) within 4 hexes of your city following your pantheon. Once you found a religion, then it's within 4 hexes of your city following your religion.
