My best guess is that the roots are rotting, and therefore the leaves that are dying aren't really connected to the plant at all.
The usual cause is pathogens in the compost. It's easy to physically damage succulent plants like the Snake Plant when you repot them. Overwatering, or irregular watering, can also physically damage the plant below ground level if the roots or leaves swell too quickly as they take up a lot of water. Any small damaged areas will let the bugs get into the plant and rot it from the inside outwards. If you have any bigger bugs or insects living in the soil, they can also cause root damage (the OP is in India - I have no idea what bugs you might find on house plants there!)
If you get the plant out of the pot and remove the soil from the roots you will probably find that the healthy roots (if any) are light coloured and the diseased ones are darker. If there are hardly any healthy roots, you might as well just throw the plant away. Otherwise, sterilize a knife (e.g. wash the blade in household bleach and then rinse the bleach off it - and don't finish by drying it on a dirty towel and put another collection of bugs on it) and cut away all the diseased roots and leaves. Then replant it in fresh compost - and preferably not from a bag of compost that somebody bought years ago, which has been sitting around accumulating pests and diseases from the environment.
You can sterilize soil and compost in a microwave. Add enough water to make sure the compost is damp, use a microwaveable container with a loose fitting lid (so any steam given off won't explode it!) and heat on full power for about 1 minute per pound of compost.
Clean the inside of the pot thoroughly. Ideally you need to sterilize it as well. If it is glazed (smooth and waterproof) on the inside, you can scrub it clean using household bleach diluted 10:1 with water, and then rinse off the bleach with running water. Otherwise, the best way is to bake the pot in an oven at about 300C for 4 hours. If you put the pot in when the oven is cold, it won't crack. And don't forget the pot will be HOT (and stay hot for a long time!) after you baked it.
The "Curry Plant" that was previously in the pot is actually a tree, and trees tend to be pretty tough at surviving diseases. Succulents like the Snake Plant aren't so tough.