The command to start mongod from command line is the same as you mentioned above
The fact that your mongod started from command is getting killed indicates your maongod is running as service
There is no need to start mongod again
When you installed it as service it is running always in the background and even after rebooting your system it comes up automatically
All you have to do is run mongo to verify whether you can connect or not
If you can connect then just run your app/node.js and continue working
Please refer to mongodb documentation for more details on how to start mongod