why did he give lydia the ricin? just to scare her not to mess with him?

I asked that too here: http://forums.d2jsp.org/topic.php?t=68911654&f=27&p=462176859

Some attempts to answer are below my post

too stupid to understand
lol'd

cmon man
if you cant figure this out, you need to stop watching television

cuz she was one of the people helping to distribute his blue meth..and he wanted everyone that was involved in his whole "meth business" to be dead so nobody would ever bother his family

One of the other bigger factors was so that his legacy lived, and that the meth he cooked turned into a legend. Walt wanted to be the one that was known for the meth.

The reason that he poisoned Lydia with the ricin is because as you hopefully saw earlier in the episode (i hope you actually watched the whole thing) Badger and Skinny Pete revealed that the Blue Meth is still on the market. Walt only taught his method to three people, Gale (dead) Jesse, and Todd. Walt believed that Jesse was dead or he was being held captive by the Nazi's and was forced to cook meth for them. Now, who is supplying the methlamine? anyone? Lydia. The reason he poisoned Lydia was because even if he couldn't have killed the nazi's he would know that their methlamine supply would no longer come in since Lydia would be dead making it so that Blue Meth was no longer on the market.

It seemed too obvious why he did that, can't believe someone didn't get this.

iLOLd at "the nazis"

lol? is this a joke question