MAD isn't even "eye for an eye" MAD is "You can blow me up, I can blow you up, if you try to blow me up I'll blow you up and if I blow you up you'll blow me up".
If I might expand on that a little, Mutual Assured Destruction doesn't quite work like that. What Assured Destruction means is that if an opponent launches a first strike, the country in possession of the Assured Destruction capability will retain enough of that capability that, after absorbing the inbound first strike, the capability to lay its enemy to waste remains viable. Normally, this is achieved by creating a nuclear force structure that is sufficiently hard enough and well-protected enough to survive an onslaught. If both sides have an Assured Destruction capability, then a state of Mutual Assured Destruction is deemed to exist.
Although colloquially MAD is referred to as a strategy, it isn't. It's an operational theory which, if its considered desirable, can be obtained by a variety of strategies. One is to make the weapons that comprise the Assured Destruction force as impossible to find as can be achieved. SSBNs are the best example of that.
Another is to so heavily protect the Assured Destruction force that it will need a disproportionately large commitment of force to disable it. This has two sub-strategies; either the weapons can be protected by piling concrete around them or by shielding them with defensive missiles. Back in the good old days, we used to calculate that the Soviets would have to fire ten warheads to be sure of disabling each of our ICBMs. That meant that we could score a 10:1 kill rate in warhead terms simply by not launching. Now you know why people MIRVed their missiles.
Another strategy for achieving Assured Destruction us to adopt launch-on-warning which means if a first strike is detected approaching, the recipient gets all the goodies off and on their way. Since, operational nuclear missiles do NOT have abort or retarget systems, once they are fired and on their way, that's it. So LoW is a very dangerous strategy. AFAIK only Pakistan actually has a formal LoW strategy.
This whole area is not a good fit for "eye for an eye". There was an effort to go that way labelled "Flexible Response" but it has really sort of faded away. The basic rule these days is "toss one at us and we'll smash your country's entire war-making capability. And that means smash EVERYTHING."