Please take an objective view of this and not emotional. Russia is here to do business and in an event of war, do what other big powers do, arm both sides. Russia supported India not out of any true love but as part of the larger game. It did not want India to fall to the US during cold war and thereby lose a country in the IOR.
The Soviets sent a sub, yes but it was part of the cold war games that the superpowers played. Plus soviets got a good market in India to sell their defense wares. Though it gave favorable terms including rupee trade, but it still made money. There were no free lunches. The soviets did for India what it had to in the game it played with the US. To call them brothers and stuff is all good, but at the end of the day, they are there to do business.
Its the game India is now playing with smaller countries in its quest to counter China. Vietnam, Myanmar, Maldives, you name it. Its not out of any love, but out of securing national and business interest. For the smaller countries, its good to play one big nation against the other as it gets them stuff either for free or at good terms.
Soviets ditched the Chinese who were politically their "brothers". So there are no brothers in international relations. Only interests. The Russians have not sold anything to Pak because they didnt want to lose Indian market. If India moves to the west to source its wares, Russia will sell to Pak and there is no doubt in that. In fact the Russians have been noticing Indias increasing closeness with the west and is opening up to Pak. Still India is a big market for them so they will stick around with us. But they have their eyes and ears on the ground to make sure they are not left behind in the game.
India is grateful for what the Soviets did as it helped us back then. But then if India was in the US's corner, it would have done the same.
Just remember that in international relations, there are no free lunches. Only permanent interests. The US is not helping India out of love but only to secure its own interests. Just that today, Indian and US interests converge and it makes sense to do business with them and call each other "allies".