Hypocrite; Ninjas

What would it have done, really?  I saw a Hobo spider yesterday on my wall, it was belaying down the wall attached to a string of web.  I went into my closet and decided which of my shoes was least important and would be OK to get spider guts all over it.  I couldn't help myself, it was like it must be done.  I murdered an animal.  I killed a spider and I did it because it made me feel fear.  Why is it that some spiders deeply and fatally harm humans?  Psychology would say that the fear of spiders, or a mere negative and startling reaction to spiders, is a long developed and adapted defense mechanism that humans have created over time with experience to the exposure of spiders.  Technically speaking, our ancestors unknowingly attempted to be friendly with spiders and received bites of fatal and gruesome consequence and since this same negative experience has happened since the dawn of time, it has become genetically conditioned within humans to be fearful of spiders - we subconsciously know that they are harmful to us.  My main question here though is, why would a spider need to be fatally harmful or just painfully harmful to humans?  What exactly is it that we are competing for that a spider would need to be able to debilitate a human?  Like many other insects, spiders drink blood - and us humans sure do have a lot of that, not to mention a strong heart that pumps it right out of any opening at a steady pace.  To what extent must we fear spiders and how do we avoid a sleepless night with one eye open staring at the spider on the ceiling that we let go, or avoid being bitten, and how do we avoid being animal murderers as well?  The role of 'image' and the 'visual' is really what plays into the fear aspect here because, not to frighten anyone but, I can only imagine how many spiders crawl across the bed or linger in close proximity on a regular basis that we don't get bitten by and don't sense fear of merely because we don't see them - we aren't aware that they are there. 

These guys are ninjas I tell you, stealthy, lethal ninjas.