Is my MacBook dead?

Sounds like you damaged it in the process.

As for which way to go, disconnecting the battery or not. It gets into which way you risk damaging your system.

Many people often just put their systems into sleep mode and not realize it so pulling the SSD or display could in fact cause a major failure of the logic board. With the older MacBook's swapping out the memory would also kill things.

So it's really important in making sure the system is really OFF and making sure you are using proper ESD protocols.

OK, so it's safer to disconnect the battery while the system is in sleep mode? NO! You will also damage the logic board! So again, you need to make sure the system is really OFF! Before you disconnect the battery.

But, now what happens when we are working on the system with the battery still plugged in and our finger slips hitting the power button? Opps! Not good either! The same could be said if you drop a screw in or short something with your screwdriver still not good!

So the bottom line here is making sure the system is really off before disconnecting the battery and being careful not the damage the connector or cable of the battery when you disconnect it. As long as you have done things correctly you should not have any issues. Again, don't forget following proper ESD protocols!

What if I'm really, really careful can I leave the battery plugged in? I personally see it as being too risky. Yes, I know people who do it and I've done it a few times too! But, I would not do it with a customers system. Also remember I've been doing this a long time so my skills are honed are yours?