Have been giving water once a week since April last week.
Weather was cold and have had a few showers in May.

For the last two three days I watered the plant regularly thinking that it needed more water since the heat has set in (June 1-2nd week).
Now after 2-3 days more leaves have turned yellow and are wilting.

Meanwhile, there were Aphids on the flowers and on some leaves.
I sprayed oil+soap mix two weeks back. No results.
So on Fri, 10th June I sprayed some insect killer and washed off the aphids.

But now the leaves are in sad state.
What is causing the Euphorbia leaves to turn yellow?

Is it shallow watering?
or aphids?
or the burns caused by the Spray?
or even ... a possibility of Salt build up ???

Please help with needed remedy.
Thanks in advance.

Photo link : https://picasaweb.google.com/hemashreesayed/EuphorbiaAmygdaloidesPurpu rea#

I'm sure it burnt by the spray. I met this case before but not on Euphorbia plants. You may try to kill aphids by hand, don't use chemical fertilizer too much and wait for new leaves.

Good luck.

Is there any hope for the plant?
If I cut off the burnt leaves from upper part of the stems, will the new leaves emerge now?

For potted plants it is relatively easy to remove aphids by holding the pot sideways under the tap and just giving the foliage a good rinse with your fingers.

I can't see in your pix what the problem is, but you mentioned it had been cold & you'd been watering too much. The combination of cold & wet can be deadly to many succulents. If it's cold then don't water.

Here's where the succulent motto comes in:

As to watering, when in doubt, don't

No one has addressed what your mix is, perhaps it's not fast draining enough. Lastly, does the pot have a drainage hole?

I'd check the drainage & then I'd leave the plant alone to recover for a while.