While i echo the OP's sentiments re a Draconian sentence, i doubt very much a life sentence will be passed in this case. None of the 3 defendants intended harm to anyone and it was their crass stupidity that caused the explosion and the subsequent death of the toddler.
The incurable mentally ill need a lifetime of care in a secure institution. It doesn't help to effectively call for vengeance, it just needs dealing with.
How much would the lead pipes in an old terrace house be worth? Just a few pounds I'd guess.
And why didn't he turn off the gas at the meter before stealing it?